Add helper to create transparent regions

[dotMorten]

See microsoft/microsoft-ui-xaml#10746 (comment) for details

[dotMorten]

Example: Create a 200x200 hole in the window, Starting at location 100,100.
We'll first create a region for the entire window, then subtract the hole:

MyWindow.SetRegion(Region.CreateRectangle(new Rect(0,0, MyWindow.Width, MyWindow.Height)) - Region.CreateRectangle(new Rect(100, 100, 200, 200)));

HolePuncher.mp4