Add InternalsVisibleTo for VS Mac

[davidwengier]

Follow up to #50389

[CyrusNajmabadi]

@sharwell to make sure this is how he wants thsi done (vs. something like EA pattern).

[davidwengier]

External access has been looked at for VS for Mac but their use of Roslyn is extensive and its too much work.

Ironically though, this is adding an IVT to an external access project, just not the one that lives in this repo. It's weird :)

[sharwell]

External access has been looked at for VS for Mac but their use of Roslyn is extensive and its too much work.

To clarify this point, VS for Mac explicitly opted out of the External Access process as part of an agreement that we are allowed to make changes that break VS for Mac, and that project is solely responsible for any resulting changes necessary to make things work again (i.e. there is no breaking changes policy on this boundary).