🐛 `NavigationAppBar`'s `automaticallyImplyLeading` behavior is wrong about fluent_ui

[bdlukaa]

In some Windows apps, the arrow is displayed even when there is no route to pop. This behavior is usually true when the app consider the navigation view as a route history.

WinUI 3 Gallery

On other apps, this behavior is not respected:

Microsoft Store

We could, indeed, add an option to customize this behavior, but it is really hard to follow a pattern when there is not a pattern.

We follow the guidance provided in the NavigationView documentation:

Any major changes to NavigationView should be made after #919 is addressed.

[mateusfccp]

You are right, I just read the documentation and there are few instructions about the back button.

Considering this documentation, I think automaticallyImplyLeading is a misleading name, specailly because it is the same name and uses the same documentation as the Material counterpart.

Instead, then, we should use the same name as in the documentation, that is, isBackButtonVisible.