elvish support for native completions

[epage]

See #3166 for more context

• code
• tests
• argcomplete and cobra can serve as examples

Tasks

• [x] #5544
• [ ] Identify and resolve any gaps with the static completions

[shannmu]

I will work on this issue, also https://github.com/clap-rs/clap/issues/3916.

[marco-m-pix4d]

Hello @epage @shannmu I read #5544 and #5744, but it is still not clear to me what is missing to have Elvish native completion. Could you please explain? Thanks :-)

[epage]

We've kept the shell support issues open because no one has yet created a definition of done for them. Mainly that identifying regressions from the existing stable completions and addressing them or at least creating issues.