Add support for tagged releases

[Tom-Hubrecht]

Currently updating a source always fetches the latest revision on the selected branch, but it may be useful to support updating through tags (c.f. https://github.com/andir/npins/blob/master/src/git.rs#L517 in npins for example).

[nikstur]

What makes this very annoying is that you need to parse and compare versions which is really hard to do in the absence of consistent versioning schemes (semantic versioning is more of a vibe than a strict rule people stick to).

[nikstur]

Maybe we could actually use the lastModified field for this: Look for all the tags and pick the newest one based on lastModified. This would make the version parsing unnecessary but could also lead to updating to e.g. pre-release versions.

[RossSmyth]

Also note that this had lead to issues in the past because sometimes projects will release security updates or continue maintaining an "LTS" release or something. For example say your project is on "12.0.0", but then the project adds a new tag for "5.1.7" to fix a security issue.