Formerly, bug-reference-setup-from-vc-alist basically had one entry
per host (like gitlab.com). Restructure so that it's easy to add new
hosts being just an instance of some type of forge such as SourceHut,
Gitea, or GitLab.
While we're at it, add support for gitea.com, salsa.debian.org, and
framagit.org, the latter two being GitLab instances.
* lisp/progmodes/bug-reference.el (bug-reference-gitea-instances)
(bug-reference-gitlab-instances,bug-reference-sourcehut-instances):
New variables listing online instances of those forges.
(bug-reference--setup-from-vc-alist): New function (and variable for
caching) using the former three new variables to generate suitable VC
auto-setup alist.
(bug-reference-try-setup-from-vc): Use both
bug-reference-setup-from-vc-alist and
bug-reference--setup-from-vc-alist.