Support applications which is using UIWindowSceneDelegate.

[arthurgau0419]

Set the windowViewController closure when you are using a UIWindowSceneDelegate based application. for example:

var config = SwiftMessages.Config()
config.windowViewController = { windowLevel, config in
    // Get my first and only window scense. 
    if let windowScense = UIApplication.shared.connectedScenes.first as? UIWindowScene {
        return WindowViewController(windowLevel: windowLevel, config: config, windowScense: windowScense)
    } else {
        return WindowViewController(windowLevel: windowLevel, config: config)
    }
}

[wtmoose]

Why not use PresentationContext.windowScene?

[arthurgau0419]

Cool, I didn't see that when I was making my changes, my SwiftMessages version is still lower then 8.0. I will close this PR. thanks!