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/HTML/MessagePort.h

@@ -53,8 +53,8 @@ public:
     void close();
 
 #undef __ENUMERATE
-#define __ENUMERATE(attribute_name, event_name)                  \
-    void set_##attribute_name(Optional<Bindings::CallbackType>); \
+#define __ENUMERATE(attribute_name, event_name)         \
+    void set_##attribute_name(Bindings::CallbackType*); \
     Bindings::CallbackType* attribute_name();
     ENUMERATE_MESSAGE_PORT_EVENT_HANDLERS(__ENUMERATE)
 #undef __ENUMERATE