LibJS: Only allocate space for Object private elements if needed

Most JavaScript objects don't have private elements, so this reduces the
size of typical JS::Object instances by two pointers.

Userland/Libraries/LibJS/Runtime/Object.h

@@ -208,7 +208,7 @@ private:
     Shape* m_shape { nullptr };
     Vector<Value> m_storage;
     IndexedProperties m_indexed_properties;
-    Vector<PrivateElement> m_private_elements; // [[PrivateElements]]
+    OwnPtr<Vector<PrivateElement>> m_private_elements; // [[PrivateElements]]
 };
 
 }