mruby-compiler: remove NODE_VARIABLE wrapper and modernize AST handling

This removes the NODE_VARIABLE enum and associated wrapper system, updating
the parser and codegen to work directly with variable-sized AST nodes.

Key changes:
- Removed NODE_VARIABLE from node.h enum
- Updated parser functions to handle direct variable-sized nodes
- Fixed codegen() main dispatch to detect variable-sized nodes directly
- Added helper functions for node type detection and header access
- Updated all parser and codegen functions to work with modern AST structure

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

mrbgems/mruby-compiler/core/node.h

@@ -85,7 +85,6 @@ enum node_type {
   NODE_HEREDOC,
   NODE_WORDS,
   NODE_SYMBOLS,
-  NODE_VARIABLE,
   NODE_LAST
 };
 
@@ -359,10 +358,6 @@ struct mrb_ast_super_node {
 #define node_to_type(x) ((enum node_type)(intptr_t)(x))
 #define node_to_char(x) ((char)(intptr_t)(x))
 
-// Macros for variable-sized nodes
-#define NODE_TYPE(n) ((enum node_type)(intptr_t)((n)->car))
-#define NODE_VAR_NODE_PTR(n) ((struct mrb_ast_var_header*)((n)->cdr))
-
 /* Phase 1 node casting macros */
 #define sym_node(n) ((struct mrb_ast_sym_node*)(n))
 #define str_node(n) ((struct mrb_ast_str_node*)(n))