Feat: Implement shell-completions using clap_complete

[titaniumtraveler]

I know having shell-completions on iamb is kinda silly considering the CLI
literally just contains --config-directory and --profile (and the short
variants), but at this point I am just too used to having them everywhere.

Usually I would also have put it into a completions subcommand, but wasn't sure if that is useful considering that there aren't any other subcommands there at the moment.

When dynamic completions get stabilized in clap, they could probably also be
used to complete available profiles and such.

---

About the cargo fmt-commit: There might be some breackage with the formatting?
fn_call_width being set anything seems to prevent rustfmt from running at
all.

So I commented that part out and cargo +nightly fmt --alled the repo.
Wasn't sure if it opening a separate PR for that was worth it.
But am totally happy to do that if wanted.

[eclairevoyant]

Not a maintainer here, but IMO formatting changes should be separated out because they cause conflicts both for the PR itself (as is currently the case) and every other PR.

[titaniumtraveler]

Not a maintainer here, but IMO formatting changes should be separated out because they cause conflicts both for the PR itself (as is currently the case) and every other PR.

Hmm, fair. I might just discard them and rebase the feat: add shell-completions using clap_complete commit.

Really just opened the PR because the missing completions annoyed my during setup lol