Add NullableReferenceTypes msbuild property

[jcouv]

Aleksey recently added a /nullable(+/-) command-line option (#30479). We also need to surface that property via Csc task and our targets file.

FYI @rainersigwald @nguerrera @davkean
We don't expect the property name "NullableReferenceTypes" would cause any conflicts.

[rainersigwald]

I agree that that seems unique enough to not worry about it

[jasonmalinowski]

@jcouv @jaredpar Was this tested with the IDE to verify it's flowing through the project system?

[jasonmalinowski]

(I guess asking @dpoeschl too who might have indeed done the testing)

[jcouv]

I was only able to test at the level of Csc task. It would be good to test end-to-end but I didn't think we have a way to test the targets file change, from last discussion with Jared.
I intended to do some validation on internal preview, but nullable feature hasn't flowed throw yet when I left, let alone this change.

[jasonmalinowski]

@jcouv @jaredpar The CompilerExtension should totally deploy that and work fine. Just build the entire solution locally and F5. Did we break that?