Introduce XRTK.WindowsXR package

[FejZa]

XRTK - Mixed Reality Toolkit Feature Request

Is your feature request related to a problem? Please describe

I am implementing a new XRTK.WindowsXR packge and leaving this one untouched, considering it legacy now. The new package will implement Windows XR support using the new OpenXR implementation and ensure compatibility with Unity 2020.3 and above series.

[SimonDarksideJ]

I completely support this. Saves any awkward backwards compatibility issues and makes it cleaner moving forward.

[StephenHodgson]

I wouldn't mind making it it's own package like com.xrtkwindows.xr or similar

[SimonDarksideJ]

I think that is @FejZa direction @StephenHodgson , using the template to create a new platform, which will effectively replace the old Windows package for 2020. The original package will remain for 2019 users