feature: help with winui 3 support

[Gitii]

feature: help with winui 3 support

[RLittlesII]

@Gitii We would gladly work with you to get what you have integrated into this package. Should be just a PR probably with a WinUI3 platform, or a separate Sextant.WinUI project. If we need to do more or something different to support it easier, we'll assess after we get your PR in.

1. It was designed that way because of simplicity and a lot of the legacy Splat container registrations use a Func implementation when you have to construct your ViewModel. I think a Lazy implementation either alongside or replacing it should be just fine. Splat doesn't have a very good Lazy story, so it was probably just to keep things simple.
2. This question is a bit more in-depth. This is really a platform-specific thing. The Xamarin Forms implementation does extend the Xamarin.Forms NavigationPage, so I think doing that for WinUI is fine.

[Gitii]

I have created the PR: #561

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