LibWeb+LibJS: Make the EventTarget hierarchy (incl. DOM) GC-allocated

This is a monster patch that turns all EventTargets into GC-allocated
PlatformObjects. Their C++ wrapper classes are removed, and the LibJS
garbage collector is now responsible for their lifetimes.

There's a fair amount of hacks and band-aids in this patch, and we'll
have a lot of cleanup to do after this.

Userland/Libraries/LibWeb/DOM/CharacterData.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibWeb/Bindings/CharacterDataPrototype.h>
 #include <LibWeb/DOM/CharacterData.h>
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOM/MutationType.h>
@@ -16,6 +17,7 @@ CharacterData::CharacterData(Document& document, NodeType type, String const& da
     : Node(document, type)
     , m_data(data)
 {
+    set_prototype(&window().ensure_web_prototype<Bindings::CharacterDataPrototype>("CharacterData"));
 }
 
 // https://dom.spec.whatwg.org/#dom-characterdata-data