feat(vcs): Add Jujutsu support

Although it uses .gitignore, Jujutsu has support for repositories without git ("non-colocated"), which we can't use "git ls-files" on

jj has its own command for listing out tracked files ("jj files"), and we can use it to determine ignored files in a similar way to Pijul

Fixes: fsfe/reuse-tool#711

[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.

fsfe / reuse-tool