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/Bindings/PlatformObject.cpp

@@ -4,10 +4,18 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/TypeCasts.h>
+#include <LibJS/Runtime/Realm.h>
 #include <LibWeb/Bindings/PlatformObject.h>
+#include <LibWeb/HTML/Window.h>
 
 namespace Web::Bindings {
 
+PlatformObject::PlatformObject(JS::Realm& realm)
+    : JS::Object(realm, nullptr)
+{
+}
+
 PlatformObject::PlatformObject(JS::Object& prototype)
     : JS::Object(prototype)
 {
@@ -15,4 +23,14 @@ PlatformObject::PlatformObject(JS::Object& prototype)
 
 PlatformObject::~PlatformObject() = default;
 
+JS::Realm& PlatformObject::realm() const
+{
+    return shape().realm();
+}
+
+HTML::Window& PlatformObject::global_object() const
+{
+    return verify_cast<HTML::Window>(realm().global_object());
+}
+
 }