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/LibJS/Heap/Cell.cpp

@@ -5,19 +5,11 @@
  */
 
 #include <LibJS/Heap/Cell.h>
-#include <LibJS/Heap/Heap.h>
-#include <LibJS/Heap/NanBoxedValue.h>
 
 namespace JS {
 
-void JS::Cell::initialize(JS::Realm&)
+void Cell::initialize(Realm&)
 {
 }
 
-void JS::Cell::Visitor::visit(NanBoxedValue const& value)
-{
-    if (value.is_cell())
-        visit_impl(value.as_cell());
-}
-
 }