💡Adding an option to use BackStackScreen without push animation

[asthatrivedi]

BackStackScreen does not have the option to push a view controller without animation like the ModalContainer does. I ran into this while implementing AlertScreen in TicTacToe sample. A clear need for this is while showing a loadingView. If you use BackStackScreen to show a loadingView, it does so with a push animation which is not inline with how LoadingViews behave in general. Currently TicTacToe has worked around it by using ModalContainer without animation but BackStackScreens should support the same.

[AquaGeek]

@asthatrivedi I think this has been addressed by the changes to make BackStack generic. If you use the same key for the loading and content screens it will update in-place.