suite: triage and promote literals/goto/events/nextvar files

[davydog187]

Summary

Triage of the Lua 5.3 official suite, fixing concrete gaps and promoting four files out of the broad :all skip:

• nextvar.lua — fully passing (was :all)
• events.lua — runs with one targeted skip range (was :all)
• goto.lua — runs with one targeted skip range (was :all)
• literals.lua — runs with six targeted skip ranges (was :all)

Remaining :all files now carry specific reasons in test/lua53_skips.exs (e.g. checkerror format mismatches, missing utf8 library, coroutines unimplemented) instead of "pending initial triage".

Behavioural fixes

• lexer: long strings drop a leading newline and normalise \r/\r\n/\n\r to \n per §3.1.
• parser: reject non-lvalue assignment targets (e.g. 0 = 1) with a syntax error instead of crashing codegen.
• scope: if/while/repeat/for bodies are now their own lexical scopes — locals declared inside no longer leak past the block (§3.3.4).
• executor __eq: Lua 5.3 selection rules — first operand's metamethod, falling back to the second's; the same-function constraint from Lua 5.1 is dropped; only consulted for table-vs-table.
• executor numeric: 0^(-n) yields the inf sentinel; NaN comparisons return false per IEEE 754.
• table.insert: raise "wrong number of arguments" with more than 3 args.
• ipairs: route lookups through __index so user metamethods drive iteration (§6.1).
• type(): raise "value expected" with no args (was returning "nil").
• debug.getinfo(N): return the calling frame's line and source via the executor's current_position + call_stack.
• math: add math.modf (preserves integer parts) and math.ult.
• string.find: handle negative init, init > #s + 1, and truthy (non-true) plain flag.
• string.byte: PUC-Lua posrelat semantics for i and j.

Test plan

• mix test — 1914 passing, 0 failures, 26 skipped (was 1907 passing before this PR)
• mix compile --warnings-as-errors
• mix format (no diff)
• mix lua.suite --status shows 4 files promoted out of :all
• New unit tests in test/lua/lexer_test.exs, test/lua/vm/metatable_test.exs, test/lua/vm/nextvar_semantics_test.exs pin each fix