Add command to generate shell completions for different shells

[dwt]

I may be missing something, but there doesn't seem to be a tool to generate shell completions as part of the command line of llm? I was expecting that as llm completions as that seems to be what has risen to the top with other tools.

That would also make installers like nix much better, as they could auto configure the correct completions for the users shell while installing.

Should that be willing, would you be willing to add that feature to llm, or is there some reason you wouldn't want that?