LibJS: Add ObjectPrototype and implement hasOwnProperty()

All Objects will now have ObjectPrototype as their prototype, unless
overridden.

Libraries/LibJS/Interpreter.h

@@ -89,6 +89,7 @@ public:
     }
 
     Object* string_prototype() { return m_string_prototype; }
+    Object* object_prototype() { return m_object_prototype; }
 
 private:
     Heap m_heap;
@@ -98,6 +99,7 @@ private:
 
     Object* m_global_object { nullptr };
     Object* m_string_prototype { nullptr };
+    Object* m_object_prototype { nullptr };
 };
 
 }