LibJS: Use a premade shape when creating iterator result objects

Instead of going through the steps of creating an empty new object,
and adding two properties ("value" and "done") to it, we can pre-bake
a shape object and cache the property offsets.

This makes creating iterator result objects in the runtime much faster.

47% speedup on this microbenchmark:

    function go(a) {
        for (const p of a) {
        }
    }
    const a = [];
    a.length = 1_000_000;
    go(a);

Userland/Libraries/LibJS/Runtime/Object.h

@@ -58,6 +58,7 @@ class Object : public Cell {
 
 public:
     static NonnullGCPtr<Object> create(Realm&, Object* prototype);
+    static NonnullGCPtr<Object> create_with_premade_shape(Shape&);
 
     virtual void initialize(Realm&) override;
     virtual ~Object();