Provide a way to bootstrap React Native Navigation into another window

[jspeth]

Experiment with integrating React Native Navigation into a hybrid app that already has its own root view controller.

• Updates the playground app delegate to create a native view controller as its root
• Extends ReactNativeNavigation to support bootstrapping into a provided window other than main
• Bootstraps into a UIWindow instance which is initially unused
• The native view controller can then access the window's root view controller to present it when desired

This is done entirely in the playground without any library changes. So this approach could be used in any app without requiring a fork of the library.

Simulator.Screen.Recording.-.iPhone.X.-.2022-03-23.at.19.06.15.mp4