LibWeb: Implement the [PutForwards] IDL extended attribute

For example, consider the attribute:

    interface Element {
        [PutForwards=value] readonly attribute DOMTokenList classList;
    }

When `classList` is set, we should instead set the attribute `value` on
the `classList` attribute of the Element interface.

Userland/Libraries/LibWeb/HTML/Window.h

@@ -233,8 +233,6 @@ private:
 
     // [[CrossOriginPropertyDescriptorMap]], https://html.spec.whatwg.org/multipage/browsers.html#crossoriginpropertydescriptormap
     CrossOriginPropertyDescriptorMap m_cross_origin_property_descriptor_map;
-
-    JS_DECLARE_NATIVE_FUNCTION(location_setter);
 };
 
 void run_animation_frame_callbacks(DOM::Document&, double now);