[BUG] navigation using NSHostingController within macOS app target does not allow for back nav

[DanBurkhardt]

AppKit applications can embed SwiftUI views, which is very handy for code-sharing between iOS and macOS targets within the same application project.

Unfortunately, this framework does not take this situation into account and implements a WindowGroup using a SwiftUI Scene, which assumes the hosting application is also a SwiftUI application.

As a result, within an AppKit / Cocoa application, we are able to successfully present the ContentView, but any navigation within the ContentView (from the example project) within a macOS app using NSHostingController does have any automatic navigation controls like the back-button, nor any other sort of method of navigating backwards.

I saw another issue opened with a similar question, but the suggestion to just use the framework as a .sheet is not an acceptable answer for a number of reasons, but mainly because it doesn't work for my application's consistent design approach.

I am fairly motivated to make this function properly, so this isn't just a complaint-- I have forked the repo and will attempt to implement support for my use case on a lightest-possible-touch basis.

@aheze will you consider this sort of improvement for review if I issue a PR?

Thanks,
Dan