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/Forward.h

@@ -217,7 +217,7 @@ class DOMStringMap;
 struct Environment;
 struct EnvironmentSettingsObject;
 class ErrorEvent;
-struct EventHandler;
+class EventHandler;
 class EventLoop;
 class HTMLAnchorElement;
 class HTMLAreaElement;
@@ -451,7 +451,6 @@ class AbortControllerWrapper;
 class AbortSignalWrapper;
 class AttributeWrapper;
 class BlobWrapper;
-struct CallbackType;
 class CanvasGradientWrapper;
 class CanvasRenderingContext2DWrapper;
 class CDATASectionWrapper;
@@ -472,7 +471,6 @@ class DOMRectReadOnlyWrapper;
 class DOMRectWrapper;
 class ElementWrapper;
 class ErrorEventWrapper;
-class EventListenerWrapper;
 class EventTargetWrapper;
 class EventWrapper;
 class FileWrapper;