Add initial GitHub actions

[wilzbach]

See #228 (comment)

Just to give an initial start.

Details:

• https://github.com/dlang-community/setup-dlang
• Advanced example: https://github.com/dlang/dmd/blob/master/.github/workflows/runnable_cxx.yml

[s-ludwig]

Hm, is it normal that some of the jobs are still pending after hours?

[wilzbach]

Hm, is it normal that some of the jobs are still pending after hours?

Not really. Though, it seems to have worked flawless on the rerun I triggered.

I have now added all jobs that are run on AppVeyor, s.t. we could already remove AppVeyor.
We probably need to see how good the through-put is before replacing Travis as there's a lot of long-running jobs there.

[wilzbach]

We'll need to update the README shield (see e.g. badges/shields#2574 (comment)), but I think this is only available once it is in the main branch.

[s-ludwig]

It says 3.x minutes, which would be awesome, although AppVeyor also did a release build. I have disabled the AppVeyor check now and made the GitHub ones required instead.