cli: add a "GitHub" output format

[woodruffw]

Closes #633.

[woodruffw]

This works, but comes with some significant limitations that make me hesitant to merge it: GitHub Actions currently limits check annotations to 10 per step, or 50 per job (if multiple steps are producing annotations). As a result, any zizmor run that produces a nontrivial number of annotations will have some annotations dropped, probably in a first-come-first serve basis.

In practice, I think this makes check annotations unworkable/unusable in the general case, since users have to dive into the action logs to see the findings that aren't given a check annotation due to the limit.

More context here: https://github.com/orgs/community/discussions/26680, https://github.com/orgs/community/discussions/68471

[woodruffw]

I'm not fully sure why, but GitHub's annotations also seem to be slightly buggy with spans that end at the end of input:

The above should have three annotations (one at the very end of the file), but that last one doesn't render.

[woodruffw]

This now works as expected:

The last thing to do here is document the limitations.