Create shell completions

[Tallented-Code-bot]

This can be done with clap; see https://dev.to/kbknapp/cli-shell-completions-in-rust-37g1.

There are two options:

• Compile time generation. This would generate the completions at compile time, and then we would have
  to distribute them in a package manager or have users install them manually.

• Runtime generation. We could add a feature/flag to jd init so that it also prints out the completion script.
  This would work well, since users would already have it sourced in their shell config.

I think that the runtime generation is the better option.