[Navigation] Avoid using reflection when accessing hardcoded regions

[nickrandolph]

What would you like to be added:

When using a TabBar or NavigationView, the associated PanelVisibilityNavigator can contain regions that are hardcoded in xaml (in contrast to a page or usercontrol that is dynamically created and added to the panel). The hardcoded regions should automatically be added as RouteMaps if they don't already exist.

Why is this needed:

In this scenario navigation should know about these regions without them being defined in a RouteMap and without using reflection to try to locate and routemap based on the path.

For which Platform:

• iOS
• Android
• WebAssembly
• WebAssembly renders for Xamarin.Forms
• Windows
• Build tasks

Anything else we need to know?