Export super types in route_data.dart library

[johnpryan]

This makes it easier to annotate routes for #3269

@TypedShellRoute<MyShellRouteData>(
  routes: <TypedRoute<RouteData>>[
    TypedGoRoute<FooRouteData>(path: '/foo'),
    TypedGoRoute<BarRouteData>(path: '/bar'),
  ],
)
class MyShellRouteData extends ShellRouteData {
  const MyShellRouteData();

  @override
  Widget builder(
    BuildContext context,
    GoRouterState state,
    Widget navigator,
  ) {
    return MyShellRouteScreen(child: navigator);
  }
}

[flutter-dashboard]

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[chunhtai]

LGTM