onBack
A callback that should be connected to any internal handling of system back done by the returned Scene.
For example, if your Scene uses a separate window that handles system back itself or if the UI present in your Scene allows users to go back via a custom gesture or affordance, this callback allows you to bubble up that event to the SceneState / io.github.alimsrepo.navease.internal.navigation.ui.NavDisplay that interfaces with the developer owned back stack.