Add GitHub Actions workflows for macOS and Linux

[hannesbraun]

These workflows compile SupMover on macOS and Linux. You'll also get the stripped binaries for download on the overview page of the build (for 90 days if I remember correctly), so you can include them in the GitHub Releases page. Currently, both workflows will run on regular amd64 machines. GitHub doesn't support ARM runners yet. I don't know how you usually would run the build on Windows, so I didn't include that. (I guess, I'd otherwise come up with a rather janky solution...)

[MonoS]

On Windows i usually just compile on my pc with whatever toolchain i've installed and upload it to the release tab.