Add support for EndeavourOS

[iguanajuice]

Title. Also working on Garuda support, but Garuda boots differently from Arch/Manjaro/EOS, so that's fun :,)

[thias]

Looks mostly good! A couple of things:

• Could you please squash the commits into one for the PR to simplify the git history?
• Looking at https://ftp.belnet.be/mirror/endeavouros/iso/ the iso regexp (EndeavourOS_[^_]+-([^-]+)\.iso) might be a bit wrong (when they have an underscore in their codename) and possible to improve. Their naming and versioning discrepancies don't help.

[iguanajuice]

• Sorry, a bit new to git. Still getting the hang of it; will do!
• Yeah, I noticed that, too. I'm hoping they keep using a dash for their codenames in the future, but I guess that's not guaranteed unless I complain about it to the eOS devs. I'll try to find a better way.