Implemented NestedWillPopScope for later handling PopScopes

[PcolBP]

I've implemented NestedWillPopScope with null-safety. Separated so if later need to be used no need to implement all data from package just this file. Updated dependency as well.

All behaviour from before stays the same.

Potential fixes of:

• #156
• #123
• #222

This PR contains no breaking change.

[ashishbeck]

Can we get this PR merged, please? It seems like a really great solution to a relatively small but difficult problem

[PcolBP]

@ashishbeck as a temporary solution before this PR will be merged, you can fork my repository and then use it in your project like this in pubspec.yaml :

persistent_bottom_nav_bar:
   git:
     url: https://github.com/PcolBP/PersistentBottomNavBar.git

[khanhtruongq]

Thanks