LibJS: Initial class implementation; allow super expressions in object

literal methods; add EnvrionmentRecord fields and methods to
LexicalEnvironment

Adding EnvrionmentRecord's fields and methods lets us throw an exception
when |this| is not initialized, which occurs when the super constructor
in a derived class has not yet been called, or when |this| has already
been initialized (the super constructor was already called).

Libraries/LibJS/Runtime/NativeFunction.h

@@ -53,7 +53,7 @@ protected:
 
 private:
     virtual bool is_native_function() const override { return true; }
-    virtual LexicalEnvironment* create_environment() override final { return nullptr; }
+    virtual LexicalEnvironment* create_environment() override final;
 
     FlyString m_name;
     AK::Function<Value(Interpreter&, GlobalObject&)> m_native_function;