Fix GH-13988: Storing DOMElement consume 4 times more memory in PHP 8.1 than in PHP 8.0

We avoid creating backing storage by using the feature introduced in
f78d5cf.

Author: ndossche

UPGRADING

@@ -1077,6 +1077,7 @@ PHP 8.4 UPGRADE NOTES
     an xpath query. This can give a time improvement of easily two order of
     magnitude for documents with tens of thousands of nodes.
   . Improved performance and reduce memory consumption of XML serialization.
+  . Reduced memory usage of node classes.
 
 - FTP:
   . Improved the performance of FTP uploads up to a factor of 10x for large

ext/dom/element.c

@@ -182,7 +182,7 @@ URL: https://dom.spec.whatwg.org/#dom-element-classlist
 */
 zend_result dom_element_class_list_read(dom_object *obj, zval *retval)
 {
-	const uint32_t PROP_INDEX = 20;
+	const uint32_t PROP_INDEX = 0;
 
 #if ZEND_DEBUG
 	zend_string *class_list_str = ZSTR_INIT_LITERAL("classList", false);

ext/dom/html_document.c

@@ -83,7 +83,7 @@ typedef struct dom_decoding_encoding_ctx {
 /* https://dom.spec.whatwg.org/#dom-document-implementation */
 zend_result dom_modern_document_implementation_read(dom_object *obj, zval *retval)
 {
-	const uint32_t PROP_INDEX = 14;
+	const uint32_t PROP_INDEX = 0;
 
 #if ZEND_DEBUG
 	zend_string *implementation_str = ZSTR_INIT_LITERAL("implementation", false);