Set up Windows CI

[exarkun]

See https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1397

None of Appveyor, Microsoft Azure, and GitHub Actions existed when that ticket was originally filed and so getting Windows CI was a lot harder. Now there are several options. Foolscap should have Windows CI of its own.

[jaraco]

I would recommend against Appveyor and Microsoft Azure. I've used all three and Github Actions is the most mature, cross-compatible, and straightforward to support.

[exarkun]

Changing the ticket title to make it clear the goal is Windows CI and Appveyor or something else is an implementation detail (and my experience is also that Appveyor is not the best option).