add a script to check the GitHub mirror

usage

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 (closed)