Add projects to migrate ViewModel code to C# (#1895)

* Rename the existing CalculatorApp.ViewModel.* namespaces to CalculatorApp.ViewModelNative.*
* Add a new C# class library project for CalculatorApp.ViewModel
* Add a new C# test project, CalculatorApp.ViewModel.Tests and run the tests in the pipeline
* To prove it all works: port the Utilities.GetWindowId function from C++ to C# and call the new version from the Calculator project and from a unit test