LibJS: Add basic prototype support

Object will now traverse up the prototype chain when doing a get().
When a function is called on an object, that object will now also be
the "this" value inside the function. This stuff is probably not very
correct, but we will improve things as we go! :^)

Libraries/LibJS/Interpreter.h

@@ -79,10 +79,20 @@ public:
     void enter_scope(const ScopeNode&, Vector<Argument>, ScopeType);
     void exit_scope(const ScopeNode&);
 
+    void push_this_value(Value value) { m_this_stack.append(move(value)); }
+    void pop_this_value() { m_this_stack.take_last(); }
+    Value this_value() const
+    {
+        if (m_this_stack.is_empty())
+            return m_global_object;
+        return m_this_stack.last();
+    }
+
 private:
     Heap m_heap;
 
     Vector<ScopeFrame> m_scope_stack;
+    Vector<Value> m_this_stack;
 
     Object* m_global_object { nullptr };
 };