Rename NODE_BEGIN to NODE_STMTS to better reflect its purpose as a
container for statement sequences, not specifically begin-end blocks.
This prepares for adding a dedicated node type for explicit begin-end
constructs.
- Rename NODE_BEGIN enum to NODE_STMTS in node.h
- Update all references in parse.y and codegen.c
- Rename new_begin function to new_stmts
Co-Authored-By: Claude <noreply@anthropic.com>
Yukihiro "Matz" Matsumoto