Add support for GitHub annotations in GitHub actions

[nijel]

• Add --github to format output according to https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions
• Add REUSE_OUTPUT_FORMAT to override output from the environment, this is useful to change output in CI compared to when running in user environment.

• [x] Added a change log entry in changelog.d/<directory>/.
• [x] Added self to copyright blurb of touched files.
• [x] Added self to AUTHORS.rst.
• [x] Wrote tests.
• [x] Documented my changes in docs/man/ or elsewhere.
• [x] My changes do not contradict the current specification.
• [x] I agree to license my contribution under the licenses indicated in the changed files.

[carmenbianca]

Wow, amazing PR @nijel ! Thank you for this work.

I wonder whether it makes sense to keep --lines in parallel to --github. They do effectively the same. For backward compatibility, we should probably keep both, though…

Do you know whether this GitHub format of errors is also consumed by other programs?

[nijel]

--lines is still more user friendly, so I'd keep both. I think gitlab has a similar format, but I have no experience with that. https://docs.astral.sh/ruff/settings/#output-format supports that in addition to GitHub...