add a script to check the GitHub mirror (!199)

## usage
```bash
nu scripts/check-mirror.nu ...[
    https://gitlab.isae-supaero.fr/dragoon/komodo
    https://github.com/dragoon-rs/komodo
    main
]
```
will
- check the Nushell version and throw a warning if mismatch
- add temporary remotes with random names
- fetch the temporary remotes
- get the revision of the `$branch` for both temporary remotes
- clean the temporary remotes
- show a message if the two revisions are not the same and how many commits they are different

> **Note**
>
> this does not conflict with #17