support 'uv --completions' to generate a completions file for bash/zsh/fish/powershell

[woutervh]

Currently uv version0.1.4 does not support tab-completions out of the box:

uv <TAB>   # does nothing

It would be nice to generate a shell-specific completions-file:

uv --completions  bash | zsh | fish | pwsh

[zanieb]

Thanks this seems reasonable to add. We'd probably want to mirror whatever we do in Ruff.