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/DocumentLoadEventDelayer.h

@@ -7,7 +7,7 @@
 #pragma once
 
 #include <AK/Noncopyable.h>
-#include <AK/NonnullRefPtr.h>
+#include <LibJS/Heap/Handle.h>
 #include <LibWeb/Forward.h>
 
 namespace Web::DOM {
@@ -21,7 +21,7 @@ public:
     ~DocumentLoadEventDelayer();
 
 private:
-    NonnullRefPtr<Document> m_document;
+    JS::Handle<Document> m_document;
 };
 
 }