mruby-compiler: optimize NODE_STMTS nesting to reduce AST depth

Modify new_stmts to flatten unnecessary nesting by returning existing
NODE_STMTS directly instead of wrapping them. This reduces memory usage
and AST complexity when multiple parentheses levels are used.

Before: (((expr1; expr2))) creates nested NODE_STMTS
After: (((expr1; expr2))) creates single NODE_STMTS with statements

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

mrbgems/mruby-compiler/core/parse.y

@@ -385,6 +385,10 @@ static node*
 new_stmts(parser_state *p, node *body)
 {
   if (body) {
+    /* If body is already a NODE_STMTS, just return it directly */
+    if (typen(body->car) == NODE_STMTS) {
+      return body;
+    }
     return list2((node*)NODE_STMTS, body);
   }
   return cons((node*)NODE_STMTS, 0);