feat: add --completions flag for shell completion generation (1)

[marcospb19]

Reopening PR #896.

(Branch for this PR was erased, but it had our license.)

[figsoda]

Don't we already generate shell completions at build time? Are there any benefits to also do this at run time?

[marcospb19]

Thanks, that's a valid point, maybe we should just leave it as it is, I mean, if you are packaging targetted for the latest commit in main you'll need to compile anyways, so you just need to set the env var at build time.

[dimpen]

This would be really helpful. I came across this issue looking for completions for zsh.
I'm installing with cargo so I don't have completions.

For example in my setup I'm using zinit for zsh and it needs a way to get the completions _ouch.
Since there's no static completions file in the repo (like completions/zsh) the next best thing is to have ouch --completions zsh.

[marcospb19]

@dimpen thanks, I'm reconsidering, provided that I manage to simplify this code and I don't see a huge binary size increase, I might include this in and switch away from build-time completions generation.