Add support for full screen covers

[ohitsdaniel]

Idea

Add support for full screen covers.

Problem description

Since iOS 14, SwiftUI supports fullscreen covers. Let's add support to NavigationNode that allows screen to be shown as a fullscreen cover. As this is not supported in iOS13, we could try to wrap the content in a FullScreenCoverTransition described in #51. For iOS14, let's just stick to what we SwiftUI provides.