Feature request: Expose screen stack events

Use case: we intend to use stacked fragments to create a card stack. A card needs to know that it's not on the top of the stack so it can render differently (for example, change its position and size, optimize away elements which won't be rendered, etc)

I'm thinking about a `NavigatorModule.addScreenStackChangedListener` or `NavigatorModule.on('screenStackChanged', (name: string, props: object) => void)` method. Then all the work to keep state of which screens are being show can be done on the JS side.

If you agree with that API, I'll (try to) make the PR.
I think implementation should not be too difficult using [FragmentManager.addOnBackStackChangedListener](https://developer.android.com/reference/android/app/FragmentManager.html#addOnBackStackChangedListener(android.app.FragmentManager.OnBackStackChangedListener)) on android and UINavigationController's delegate `didShowViewController`.

Start by reviewing NavigatorModule and the existing screen-stack handling on Android and iOS. Compare the proposed FragmentManager back-stack listener and UINavigationController delegate callback, then define completion as exposing screen-stack changes to JavaScript for stacked cards.

Written by the indexing model from the issue text.