Mark WP_Parser_Node as final

Nothing extends WP_Parser_Node. Marking it final lets PHP's opcache
and tracing JIT specialize property access and method dispatch since
the class layout is now fixed. Small but consistent improvement
measured across multiple runs under tracing JIT (~+2% avg, ~+2% best).

End-to-end parser benchmark:
  tracing JIT: ~57K -> ~57-58K QPS avg, 60-61K QPS best
  no JIT:      ~33K -> ~34K QPS avg, 35K QPS best

Author: JanJakes

packages/mysql-on-sqlite/src/parser/class-wp-parser-node.php

@@ -9,7 +9,7 @@
  * In this way, a parser node constitutes a recursive structure that represents
  * a parse (sub)tree at each level of the full grammar tree.
  */
-class WP_Parser_Node {
+final class WP_Parser_Node {
 	/**
 	 * @TODO: Review and document these properties and their visibility.
 	 */