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/Forward.h

@@ -159,6 +159,7 @@ class BigInt;
 class BoundFunction;
 struct CachedSourceRange;
 class Cell;
+class CellImpl;
 class CellAllocator;
 class ClassExpression;
 struct ClassFieldDefinition;