Added string operations native to NAS

[MofumofuChan]

As a language where all packages are strings and string manipulation and interpretation is a core part of the language, a rich variety of string-related operations would greatly benefit the language's usability. As such, I have implemented a few operations:

//    setstrindexof [package] [string]
//    Finds the first zero-based index instance of the string in the package and sets that to [package].IndexOf. If none is found, it's set to -1.
//    For example:
//        set someString abc zbc
//        setstrindexof someString bc
//        someString.IndexOf = 1
//
//    setstrlastindexof [package] [string]
//    Finds the last zero-based index instance of the string in the package and sets that to [package].IndexOf. If none is found, it's set to -1.
//    For example:
//        set someString abc zbc
//        setstrindexof someString bc
//        someString.IndexOf = 5
//
//    setstrremove [package] [string]
//    Removes the first instance of [string] from [package]. If [string] is not contained in [package], nothing happens.
//    For example:
//        set someString Everyday thing
//        setstrremove someString day
//        someString = "Every thing"
//
//    setstrstartswith [package] [string]
//    Sets [package].StartsWith to true [package] starts with [string], otherwise sets it to false.
//    For example:
//        set someString Everyday thing
//        setstrstartswith someString Every
//        someString.StartsWith = "true"
//
//    setstrendswith [package] [string]
//    Sets [package].EndsWith to true [package] ends with [string], otherwise sets it to false.
//    For example:
//        set someString Everyday thing
//        setstrendswith someString thing (In the documentation I accidentally wrote setstrstartswith instead of setstrendswith and only noticed it now)
//        someString.EndsWith = "true"
//
//    setstrupper [package]
//    Sets [package] to uppercase.
//    For example:
//        set someString Everyday thing
//        setstrupper someString
//        someString = "EVERYDAY THING"
//
//    setstrlower [package]
//    Sets [package] to lowercase.
//    For example:
//        set someString Everyday thing
//        setstrlower someString (Again, accidentally wrote setstrupper)
//        someString = "everyday thing"
//
//    setstrinsert [package] [start] [string]
//    Inserts [string] in [package] at the zero-based index [start].
//    For example:
//        set someString Everyday thing
//        setstrinsert someString 3 WOW
//        someString = "EveWOWryday thing"
//
//    setstrsubstring [package] [start] <length>
//    Gets the substring of [package] starting at the zero-based index [start] and going until the end or until a desired <length> and sets that to [package].Substring. (Forgot to add a period at the end)
//    For example:
//        set someString Everyday thing
//        setstrsubstring someString 5 7
//        someString.Substring = "day thi"
//
//    setstrreverse [package]
//    Reverses [package].
//    For example:
//        set someString Everyday thing
//        setstrreverse someString
//        someString = "gniht yadyrevE"
//
//    setstrbefore [package] [string]
//    Gets the substring of [package], starting from the beginning and ending at the first instance [string], not including [string], and sets that to [package].Before.
//    For example:
//        set someString Everyday thing
//        setstrbefore someString day
//        someString.Before = "Every"
//
//    setstrafter [package] [string]
//    Gets the substring of [package], starting from the first instance of [string], not including [string], going all the way to the end, and sets that to [package].After.
//    For example:
//        set someString Everyday thing
//        setstrafter someString day
//        someString.After = " thing"
//
//    setisnumber [package]
//    If [package] is a number, set [package].IsNumber to true, otherwise set it to false.

This is the NAS file used to test all of these features:

#Test
    set hello press stress
    setstrindexof hello ss
    msg First index of ss in "{hello}": {hello.indexof}
    setstrlastindexof hello ss
    msg Last index of ss in "{hello}": {hello.indexof}
    setstrindexof hello ssasd
    msg First index of ssasd in "{hello}": {hello.indexof}
    setstrindexof hello
    msg First index of in "{hello}": {hello.indexof}

    setstrremove hello e
    msg "{hello}" without the first e: {hello}
    set hello press stress
    setstrremove hello
    msg "{hello}" without the first: {hello}
    set hello press stress
    setstrremove hello owo
    msg "{hello}" without the first owo: {hello}
    set hello press stress

    setstrstartswith hello press
    msg "{hello}" starts with press: {hello.startswith}
    setstrstartswith hello owo
    msg "{hello}" starts with owo: {hello.startswith}
    setstrstartswith hello
    msg "{hello}" starts with: {hello.startswith}
    setstrendswith hello stress
    msg "{hello}" ends with press: {hello.endswith}
    setstrendswith hello owo
    msg "{hello}" ends with owo: {hello.endswith}
    setstrendswith hello
    msg "{hello}" ends with: {hello.endswith}

    set hello Press Stress
    set hello.changed {hello}
    setstrupper hello.changed
    msg "{hello}" to uppercase: {hello.changed}
    setstrlower hello.changed
    msg "{hello}" to lowercase: {hello.changed}

    set hello.changed {hello}
    setstrinsert hello.changed 3 WOAH
    msg "{hello}" with "WOAH" at index 3: {hello.changed}
    set hello.changed {hello}
    setstrinsert hello.changed 3
    msg "{hello}" with "" at index 3: {hello.changed}

    setstrsubstring hello 3
    msg "{hello}" starting from index 3: {hello.substring}
    setstrsubstring hello 3 8
    msg "{hello}" starting from index 3 and length 8: {hello.substring}
    setstrsubstring hello 0 0
    msg "{hello}" starting from index 0 and length 0: {hello.substring}

    set hello.changed {hello}
    setstrreverse hello.changed
    msg "{hello}" reversed: {hello.changed}

    setstrbefore hello tre
    msg "{hello}" before "day": {hello.before}
    setstrafter hello tre
    msg "{hello}" after "day": {hello.after}

    set hello 10
    setisnumber hello
    msg Is "{hello}" a number? {hello.isnumber}
    set hello 2.3
    setisnumber hello
    msg Is "{hello}" a number? {hello.isnumber}
    set hello -1
    setisnumber hello
    msg Is "{hello}" a number? {hello.isnumber}
    set hello -4.6
    setisnumber hello
    msg Is "{hello}" a number? {hello.isnumber}
    set hello inf
    setisnumber hello
    msg Is "{hello}" a number? {hello.isnumber}
    set hello -inf
    setisnumber hello
    msg Is "{hello}" a number? {hello.isnumber}
    set hello 0
    setisnumber hello
    msg Is "{hello}" a number? {hello.isnumber}
    set hello owo
    setisnumber hello
    msg Is "{hello}" a number? {hello.isnumber}
    set hello
    setisnumber hello
    msg Is "{hello}" a number? {hello.isnumber}
quit

#TestInsert1
    set hello Press Stress
    setstrinsert hello
quit

#TestInsert2
    set hello Press Stress
    setstrinsert hello what is this?
quit

#TestInsert3
    set hello Press Stress
    setstrinsert hello -1 what is this?
quit

#TestInsert4
    set hello Press Stress
    setstrinsert hello 100 what is this?
quit

#TestSubstring1
    set hello Press Stress
    setstrsubstring hello
quit

#TestSubstring2
    set hello Press Stress
    setstrsubstring hello owo
quit

#TestSubstring3
    set hello Press Stress
    setstrsubstring hello -1
quit

#TestSubstring4
    set hello Press Stress
    setstrsubstring hello 100
quit

#TestSubstring5
    set hello Press Stress
    setstrsubstring hello 3 wha
quit

#TestSubstring6
    set hello Press Stress
    setstrsubstring hello 3 -1
quit

#TestSubstring7
    set hello Press Stress
    setstrsubstring hello 3 10
quit

#TestBefore1
    set hello Press Stress
    setstrbefore hello owo
quit

#TestAfter1
    set hello Press Stress
    setstrafter hello owo
quit