Add UseWinUI property

[huichen123]

We will use this property to identify WinUI project.

[dotMorten]

I'm curious what this property does, since there are no ms build changes?
Would have been cleaner that the WinUI package set this for you if it was needed (but that would require nuget dependency hierarchy to be more logically ordered, so you don't get the WinUI package transitively like it is now - see #893)

[huichen123]

I'm curious what this property does, since there are no ms build changes?
Would have been cleaner that the WinUI package set this for you if it was needed (but that would require nuget dependency hierarchy to be more logically ordered, so you don't get the WinUI package transitively like it is now - see #893)

The property will be used to exclude WinUI stuff from those not interesting in WinUI. The build change is targeted for 1.0. Getting this property in first so that 0.8 can be upgraded to 1.0 (i am not saying there will be no other breaking change though).

[evelynwu-msft]