LibWeb: Move event listeners, handlers and callbacks to the GC heap

This patch moves the following things to being GC-allocated:
- Bindings::CallbackType
- HTML::EventHandler
- DOM::IDLEventListener
- DOM::DOMEventListener
- DOM::NodeFilter

Note that we only use PlatformObject for things that might be exposed
to web content. Anything that is only used internally inherits directly
from JS::Cell instead, making them a bit more lightweight.

Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.cpp

@@ -10,7 +10,7 @@
 namespace Web::ResizeObserver {
 
 // https://drafts.csswg.org/resize-observer/#dom-resizeobserver-resizeobserver
-NonnullRefPtr<ResizeObserver> ResizeObserver::create_with_global_object(JS::GlobalObject& global_object, Bindings::CallbackType const& callback)
+NonnullRefPtr<ResizeObserver> ResizeObserver::create_with_global_object(JS::GlobalObject& global_object, Bindings::CallbackType* callback)
 {
     // FIXME: Implement
     (void)global_object;