[Cupertino] Add unfinished Navigation gallery demo.

Author: IIzzaya

Runtime/cupertino/route.cs

@@ -239,7 +239,7 @@ public override void paint(Canvas canvas, Offset offset, ImageConfiguration conf
     public class CupertinoPageRoute : PageRoute {
         public CupertinoPageRoute(
             WidgetBuilder builder,
-            RouteSettings settings,
+            RouteSettings settings = null,
             string title = "",
             bool maintainState = true,
             bool fullscreenDialog = false