Xonsh shell support in rustup

[anki-code]

Problem you are trying to solve

Hey!
It will be cool to have xonsh shell support.
Thanks!

Solution you'd like

I would like to see the xonsh shell in the list of shells:

Notes

No response

[djc]

Would you be able to submit a PR?

[anki-code]

hi @djc! I believe yes if you could drop a few pointers/recommendations.

[djc]

Start here: https://github.com/rust-lang/rustup/blob/main/src/cli/self_update/shell.rs#L63.