add a Nushell version update script and bump all versions to `0.81.0`

[amtoine]

related to

Thanks for the PR! You forgot to update the dependencies 🥈 lol

Don't worry, I am going to pump the dependencies after this #18

Originally posted by @AucaCoyan in https://github.com/nushell/nufmt/issues/20#issuecomment-1583078158

ok so i've written a script to automate the process of changing the version of nushell/nufmt to avoid ending up in the same situation as in #20 :grimacing:

[AucaCoyan]

Excellent! Nice work! Have your medal 🥇 🌟 🎆

[fdncred]

Thanks