feat: add shell completion generation

[NickyMeuleman]

This adds the ability to generate shell completions to the CLI.

While this PR is not in an ideal form yet, I'd be more than happy to work together to make it fit the used patterns in this repo.
I tried following them, but ran into trouble because the execute method for each command did not get a Command passed, which this shell completions generator needs.

Here is an example of me using the completions this generates:
https://asciinema.org/a/dsXad2IayVyXJFHKX0GjvRhjH

[loewenheim]

This is very cool, thank you for the contribution! I'll get back to you on Monday to work out the kinks.

[loewenheim]

Another thing—did you intentionally move use statements around in 57890f1? I'd rather they stay as they were.

ETA: You will also need to fix the command_help test. The easiest way to do this is to run TRYCMD=overwrite cargo test. This will update all test snapshots with the new output. You can then check that output and commit it.

Other than that, this is ready to merge, I think.

[NickyMeuleman]

Ah, I turned on format-on-save before making that commit, that must have reordered those use statements.
Put them back, and updated the snapshot file!

Do I need to write some docs for this also?

Thanks for the help and patience (with my not knowing how a CLI actually works)

[NickyMeuleman]

I saw the Windows test failed, because they are also missing an update to the snapshot tests.

So I tried to set up this repo on a Windows machine, but 2 other tests failed there on the main branch. And curiously, those tests didn't fail in CI, but only when running cargo test locally on the Windows machine.

[NickyMeuleman]

The errors happened because I used regular Windows cmd, when I used git bash, everything worked.
I updated the Windows snapshot test that was failing, all checks should pass now.

[loewenheim]

Merged! Thank you very much for the contribution!