Add tooling to ensure no API changes are made in patch updates

[natemcmaster]

Follow-up to #3610

We should add tooling so we can ensure that 3.0.1, 3.0.2, etc do not change any public API in the reference assemblies.

[Pilchie]

@dougbu can you take a look at this? Or maybe @anurse can nominate someone else?

[analogrelay]

I'm not sure this is something we'd have resources to support in preview 8 given the very short timeline. We could look at it as part of infrastructure work in preview 9 though since getting infra-/test-only changes through ask-mode should be much simpler. We already have some things we want to do to improve API changes all up, so this would be part of that work.

[Pilchie]

Fair enough, I'll move to Preview 9