feat(ast): export extern/export function flags

The LSP semantic checks need FLAG_IS_EXTERN/FLAG_IS_EXPORT to skip declaration-only functions.

ast/__init__.vp

@@ -12,6 +12,7 @@ from .base import (
     # Constants
     CONST_INT, CONST_FLOAT, CONST_STR, CONST_BOOL, CONST_NONE, CONST_CHAR,
     FLAG_IS_ASYNC, FLAG_SIMPLE, FLAG_HAS_STAR, FLAG_IS_DEFINE,
+    FLAG_IS_EXTERN, FLAG_IS_EXPORT,
     # Helper Functions
     _init_ast, _copy_str, _set_pos, _inherit_pos, _set_parent_list,
     _binop_from_op, _augop_from_op, _cmpop_from_op,