add support for Custom controls (C# UWP project)

[Samuel12321]

This project could be quite useful with support for Custom controls (C# UWP project)

[nesherhh]

I have tried but did not succeeded. As far as I understand, msbuild tasks that process xaml and resources are built for UWP. They can't be run in .NET 5. I would be glad to be wrong about this. If you know how to do this you are welcome to submit PR.