Add ReactiveApp.Navigation

This PR moves the `ReactiveShell/View/AppBar` navigation stack into a separate project to decouple it from the other parts of the framework. This also allows us to switch navigation stack if we want without having a large impact on the other parts of the project.

## Goal

Decouple `ReactiveShell/View/AppBar` from the rest of the framework. This makes the transition to this new framework much easier for existing applications.

## Other changes

None.

## TODO:

- [ ] Add the projects for the supported platforms.
- [ ] Move classes.
- [ ] Add necessary extension points to ReactiveApp.