Support for Gitea/Forgejo

[pieterhijma]

There is support for github-latest-release and gitlab-latest-release but there is no similar support for Gitea or Forgejo (a soft fork of gitea). An example of an important Forgejo instance is Codeberg.org, currently with 14 projects in Quicklisp.

It would be nice if source.txt could contain gitea-latest-release in a similar way.

I will make a pull-request for this.