Add support for predictive back

[dfabulich]

https://developer.android.com/develop/ui/compose/system/predictive-back
https://developer.android.com/guide/navigation/custom-back/predictive-back-gesture
https://developer.android.com/design/ui/mobile/guides/patterns/predictive-back

We're currently using a BackHandler, which prevents predictive back animations.

skip-ui/Sources/SkipUI/SkipUI/Containers/Navigation.swift

Lines 261 to 266 in b98d3a2

	// Intercept system back button to keep our state in sync
	BackHandler(enabled: !arguments.isRoot) {
	if arguments.toolbarPreferences.backButtonHidden != true {
	navigator.value.navigateBack()
	}
	}

[dfabulich]

I spent some time investigating this, and got pretty stuck. There's just no good place to handle the SwiftUI back stack without using BackHandler.

I have a hunch we'll have to upgrade to Navigation 3 to get this to work properly. #292

[marcprux]

Fixed by #377