LibJS: Move native objects towards two-pass construction

To make sure that everything is set up correctly in objects before we
start adding properties to them, we split cell allocation into 3 steps:

1. Allocate a cell of appropriate size from the Heap
2. Call the C++ constructor on the cell
3. Call initialize() on the constructed object

The job of initialize() is to define all the initial properties.
Doing it in a second pass guarantees that the Object has a valid Shape
and can find its own GlobalObject.

Libraries/LibJS/Runtime/Object.h

@@ -57,6 +57,7 @@ public:
     static Object* create_empty(Interpreter&, GlobalObject&);
 
     explicit Object(Object* prototype);
+    virtual void initialize(Interpreter&, GlobalObject&) override;
     virtual ~Object();
 
     enum class GetOwnPropertyMode {