LibWeb: Retain calculated Element::ordinal_value for lists

`Element::ordinal_value` is called for every `li` element in
a list (ul, ol, menu).

Before:
  `ordinal_value` iterates through all of the children of the list
  owner. It is called once for each element: complexity $O(n^2)$.

After:
  - Save the result of the first calculation in `m_ordinal_value`
  then return it in subsequent calls.
  - Tree modifications are intercepted and trigger invalidation
    of the first node's `m_ordinal_value`:
    - insert_before
    - append
    - remove
  Results in noticeable performance improvement rendering' large
  lists: from 20s to 4s for 20K elements.

Libraries/LibWeb/HTML/HTMLLIElement.cpp

@@ -34,6 +34,7 @@ void HTMLLIElement::attribute_changed(FlyString const& local_name, Optional<Stri
     if (local_name == HTML::AttributeNames::value) {
         if (auto* owner = list_owner()) {
             owner->set_needs_layout_tree_update(true, DOM::SetNeedsLayoutTreeUpdateReason::HTMLOListElementOrdinalValues);
+            maybe_invalidate_ordinals_for_list_owner();
         }
     }
 }