Base OpenURL dependency on SwiftUI

[tgrapperon]

This simplifies the implementation of the OpenURL dependency and should allow to build dynamic frameworks with APPLICATION_EXTENSION_API_ONLY.

This should address #1673.

As a consequence, this raises the minimal availability of this dependency to iOS 14 and macOS 11.

[stephencelis]

This looks good to us, thanks! It's relatively new, so hopefully not too many folks rely on it for iOS 13, and if they do it's hopefully easy enough to fix with a custom dependency.