LibJS: Don't directly teach the heap about the javascript VM or Realm

Instead, smuggle it in as a `void*` private data and let Javascript
aware code cast out that pointer to a VM&.

In order to make this split, rename JS::Cell to JS::CellImpl. Once we
have a LibGC, this will become GC::Cell. CellImpl then has no specific
knowledge of the VM& and Realm&. That knowledge is instead put into
JS::Cell, which inherits from CellImpl. JS::Cell is responsible for
JavaScript's realm initialization, as well as converting of the void*
private data to what it knows should be the VM&.

Libraries/LibWeb/HTML/AnimationFrameCallbackDriver.h

@@ -9,6 +9,7 @@
 #pragma once
 
 #include <AK/HashMap.h>
+#include <LibJS/Heap/Cell.h>
 #include <LibJS/Heap/GCPtr.h>
 #include <LibJS/Heap/HeapFunction.h>
 #include <LibWeb/WebIDL/Types.h>