Cover five missing language constructs identified from comparison with
the existing tree-sitter-rune crate by zhuhaow:
- impl_item: impl blocks with optional name and visibility
- const_item: const declarations with visibility support
- try_expression: postfix ? operator at precedence 13
- loop_label/label: 'name: labels on loop/while/for, referenced in
break/continue
- object_pattern/object_pattern_entry: #{ key: pat } destructuring in
match with shorthand, string keys, and rest patterns
All 55 tests pass (28 existing + 27 new).
vikingowl
9b7625de68