Add jujutsu (jj) completions (#1177)

I have written custom completions for
[Jujutsu](https://jj-vcs.github.io/jj/), a modern version control system
(vcs). It uses git as a backend, making it compatible with git-based
tools.

Jujutsu provide official completions for Nushell, but these offer more
features. Specifically, it can auto-complete revisions, bookmarks,
remotes, or operations when appropriate.

Example: 
<img width="1408" height="908" alt="image"
src="https://github.com/user-attachments/assets/a2b988e9-67c4-4a4f-961b-865b66e8f8cb"
/>

Author: alightgoesout