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/MutationObserver.cpp

@@ -12,7 +12,7 @@
 namespace Web::DOM {
 
 // https://dom.spec.whatwg.org/#dom-mutationobserver-mutationobserver
-MutationObserver::MutationObserver(Bindings::WindowObject& window_object, Bindings::CallbackType callback)
+MutationObserver::MutationObserver(Bindings::WindowObject& window_object, JS::Handle<Bindings::CallbackType> callback)
     : m_callback(move(callback))
 {
     // 1. Set this’s callback to callback.