Add GitHub Actions workflow to generate preview tarballs for PRs

[link2xt]

Now that a scripts/dist.sh script to generate release tarballs is added in #38, we can add a GHA workflow to generate release tarballs for each PR.

Workflow should be triggered on pull request, upload the artifact and post a link to the artifact as a status or the comment. To post statuses or comments actions/github-script can be used.

[Septias]

Can you care about this @link2xt?

[link2xt]

Yes, it's on my todo list but with some things in front of it

[Septias]

Nice :)