TODO.md: remove pattern matching (implemented)

pattern matching is now implemented with support for:
- case/in syntax with multiple in-clauses
- array patterns with rest (*) and post-rest elements
- hash patterns with shorthand and rest (**)
- guard clauses (if/unless)
- alternative patterns (|)
- pin operator (^)
- as pattern (=>)
- one-line pattern matching (expr in pat, expr => pat)
- NoMatchingPatternError exception

Co-authored-by: Claude <noreply@anthropic.com>

TODO.md

@@ -3,7 +3,6 @@
 # After mruby 3.4
 
 - parser and code generator independent from `mrb_state` (picoruby?)
-- pattern matching (case/in syntax, array/hash patterns, guards, etc.)
 - iv/hash entry cache
 - method inline caching improvements (cache method lookup results)
 - more peephole optimization (if possible)