mruby-compiler: add brace-less hash pattern support

Add support for brace-less hash patterns at top level of case/in.
`in a: x, b: y` is now equivalent to `in {a: x, b: y}`.
`in a:, b:` shorthand now works with newlines (CRuby compatible).

Changes:
- Add EXPR_VALUE to IS_LABEL_POSSIBLE() to recognize labels after `in`
- Add brace-less hash pattern rules to p_expr
- Change p_hash_elem to use p_as instead of p_expr to avoid recursion
- Add in_kwarg flag to parser state for pattern matching context
- Set in_kwarg in lexer when keyword_in is returned
- Use EXPR_ARG after tLABEL_TAG when in_kwarg is set (makes newlines significant)

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

include/mruby/compile.h

@@ -103,7 +103,7 @@ struct mrb_parser_state {
   unsigned int cmdarg_stack;
   int paren_nest;
   int lpar_beg;
-  int in_def, in_single;
+  int in_def, in_single, in_kwarg;
   mrb_bool cmd_start:1;
   mrb_ast_node *locals;