LibWeb: Don't invalidate style for entire document on attribute change

We now only invalidate the style of the context element and all of its
descendants. It's still very aggressive, but much less than before.

Note that this will need to become a lot smarter once we implement the
CSS :has() selector.

Userland/Libraries/LibWeb/DOM/Element.h

@@ -173,6 +173,8 @@ protected:
 private:
     void make_html_uppercased_qualified_name();
 
+    void invalidate_style_after_attribute_change(FlyString const& attribute_name);
+
     WebIDL::ExceptionOr<JS::GCPtr<Node>> insert_adjacent(String const& where, JS::NonnullGCPtr<Node> node);
 
     QualifiedName m_qualified_name;