Migrate from AppVeyor to GitHubActions

[knocte]

Fixes #863

[knocte]

@Thorium ping?

(Note that we were planning to expand this with macOS&Linux runners, but wanted to get this PR merged first, since AppVeyor was only doing Windows builds anyway.)

[knocte]

@Thorium mmm, even if this PR removed appveyor.yml, I still see its status in the CI icon of the master branch. I wonder if you have to delete the link/hook in the repo settings, or in the AppVeyor website?

[knocte]

I wonder if you have to delete the link/hook in the repo settings, or in the AppVeyor website?

@Thorium Seems my suspicion was correct? see:

[Thorium]

Ok marked them not Active.

[knocte]

@Thorium thanks! I opened next PR here: #865

And the next one after that will be expanding CI to more OSs.