SkipUI should use Compose `WindowInsets` instead of a `_safeArea` environment value

[dfabulich]

https://developer.android.com/develop/ui/compose/system/evaluate-rulers

WindowInsets is the standard API in Jetpack Compose for handling areas of the screen that are partially or fully obscured by the system UI. These areas include the status bar, navigation bar, and on-screen keyboard.

Currently, SkipUI manages the safe area using an internal _safeArea environment value. This creates a lot of tricky busy work, especially in NavigationStack and TabView, making sure that SkipUI's _safeArea lines up with WindowInsets provided by the system.

It would be simpler and better to make SkipUI code work with WindowInsets, adding/consuming padding as we go.