feat: add keyword subcategories to Ruby highlights (#275)

## Summary

Splits the monolithic `@keyword` capture in Ruby's `highlights.scm` into
semantic subcategories, enabling themes to style different keyword types
independently.

### New captures

| Capture | Keywords |
|---------|----------|
| `@keyword.function` | `class`, `def`, `module` |
| `@keyword.control.conditional` | `if`, `elsif`, `else`, `unless`,
`case`, `when`, `then` |
| `@keyword.control.repeat` | `while`, `until`, `for`, `do` |
| `@keyword.control.return` | `return`, `next`, `break`, `retry`,
`yield` |
| `@keyword.exception` | `begin`, `ensure`, `rescue` (alongside existing
`raise`/`fail`/`catch`/`throw`) |
| `@keyword` | `alias`, `and`, `end`, `in`, `or` (general/structural) |

### Backward compatibility

Fully backward compatible — themes that only define `keyword` will
continue to work via Zed's fallback resolution (e.g.
`keyword.control.return` → `keyword.control` → `keyword`).

### Precedent

Follows the pattern already established by the SCSS extension, which
uses `keyword.control.conditional`, `keyword.control.return`,
`keyword.function`, and `keyword.repeat`.

### Motivation

Currently it's impossible to visually distinguish `return`/`yield` from
`def`/`end` or `if`/`unless` in Ruby. This is a common theme
customisation need — other editors (VS Code, Sublime) support this level
of granularity. This change brings Ruby in line with other Zed language
extensions that already provide keyword subcategories.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: infinityrobot

languages/ruby/highlights.scm

@@ -4,35 +4,55 @@
   (global_variable)
 ] @variable
 
-; Keywords
+; Keywords — definition
 [
-  "alias"
-  "and"
-  "begin"
-  "break"
-  "case"
   "class"
   "def"
-  "do"
+  "module"
+] @keyword.function
+
+; Keywords — conditional
+[
+  "case"
   "else"
   "elsif"
-  "end"
-  "ensure"
-  "for"
   "if"
-  "in"
-  "module"
-  "next"
-  "or"
-  "rescue"
-  "retry"
-  "return"
   "then"
   "unless"
-  "until"
   "when"
+] @keyword.control.conditional
+
+; Keywords — loop
+[
+  "do"
+  "for"
+  "until"
   "while"
+] @keyword.control.repeat
+
+; Keywords — flow return/jump
+[
+  "break"
+  "next"
+  "retry"
+  "return"
   "yield"
+] @keyword.control.return
+
+; Keywords — exception handling
+[
+  "begin"
+  "ensure"
+  "rescue"
+] @keyword.exception
+
+; Keywords — general
+[
+  "alias"
+  "and"
+  "end"
+  "in"
+  "or"
 ] @keyword
 
 ((identifier) @keyword