LibWeb/HTML: Add fastpath to skip selectedness update on children change

We can definitely expand on this a bunch more, but using the metadata
provided in the children change notification we are able to skip
runnning the expensive selectedness algorithm on the <select> element.

This removes children changed from appearing in the profile of:

https://wpt.live/html/select/options-length-too-large.html

Libraries/LibWeb/HTML/HTMLSelectElement.h

@@ -110,6 +110,7 @@ private:
     virtual void computed_properties_changed() override;
 
     virtual void children_changed(ChildrenChangedMetadata const*) override;
+    bool can_skip_children_changed_selectedness_update(ChildrenChangedMetadata const& metadata) const;
 
     void update_cached_list_of_options() const;
     void show_the_picker_if_applicable();