LibJS: Add JS_CELL macro and use it in all JS::Cell subclasses

This is similar to what we already had with JS_OBJECT (and also
JS_ENVIRONMENT) but sits at the top of the Cell inheritance hierarchy.

Userland/Libraries/LibJS/Runtime/Array.h

@@ -35,7 +35,6 @@ public:
         return Array::create_from(realm, values);
     }
 
-    explicit Array(Object& prototype);
     virtual ~Array() override = default;
 
     virtual ThrowCompletionOr<Optional<PropertyDescriptor>> internal_get_own_property(PropertyKey const&) const override;