Add WinUI 3 Branch

We should have a branch of this repo which shows how to do this same technique with WinUI 3 and C#/WinRT.

It theoretically should be easier to accomplish hopefully? But not sure as haven't tried any of that yet. If it's significantly different I'm not sure if we want it as it's own sample repo. For now though, branching it seems like a good start to experiment.

Review the existing repository sample and identify the technique this issue wants reproduced. Investigate whether WinUI 3 with C#/WinRT differs substantially from the current C++/WinRT UWP sample, then determine whether a branch or separate sample repository is appropriate. Done means the approach and repository location are decided and the WinUI 3 sample is implemented if the scope is viable.

Written by the indexing model from the issue text.