Format with `cargo fmt`

[9999years]

My editor is configured to format on save, which I have to disable in the broot repo to avoid making changes to parts of files I'm not modifying. It would be simpler as a contributor if I could just format my code automatically. Happy to put up a PR + CI for this.

[Canop]

Sorry, I add automatic fmt on repos when I can (see for example bacon), but cargo fmt is way too bad, even in nightly, for broot today.

I don't need a PR for that, I follow the progress of cargo fmt.