LibWeb: Re-implement HTML::Navigator using IDL

Get rid of the bespoke NavigatorObject class and use the modern IDL
strategies for creating platform objects to re-implement Navigator and
its associcated mixin interfaces. While we're here, implement it in a
way that brings WorkerNavigator up to spec :^)

Userland/Libraries/LibWeb/HTML/Window.h

@@ -157,6 +157,7 @@ private:
     JS::GCPtr<HighResolutionTime::Performance> m_performance;
     JS::GCPtr<Crypto::Crypto> m_crypto;
     JS::GCPtr<CSS::Screen> m_screen;
+    JS::GCPtr<HTML::Navigator> m_navigator;
 
     AnimationFrameCallbackDriver m_animation_frame_callback_driver;