feat: Default `Routes` in `RouteLayers`

[Siraj-Aizlewood]

Users have expressed an interest in having default Routes in Route Layers. That is, if no other Routes are chosen, then use the default.

While this can be achieved manually in the code by simply checking to see if none of the Routes are returned by the RouteLayer it would be convenient to have default Routes baked into the RouteLayer. Particularly if the RouteLayer is being saved and shared.