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/DOM/AbortSignal.h

@@ -47,7 +47,7 @@ public:
 
     void signal_abort(JS::Value reason);
 
-    void set_onabort(Optional<Bindings::CallbackType>);
+    void set_onabort(Bindings::CallbackType*);
     Bindings::CallbackType* onabort();
 
     // https://dom.spec.whatwg.org/#dom-abortsignal-reason