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/Forward.h

@@ -304,6 +304,7 @@ class MessageChannel;
 class MessageEvent;
 class MessagePort;
 struct NavigationParams;
+class Navigator;
 class Origin;
 class PageTransitionEvent;
 class Path2D;