FR: PowerShell completion

[WardBrian]

Especially with Windows support growing, this would be a real nice-to-have.

The easiest way to get it might be bumping the vendored cmdliner and applying the patch mentioned here to preserve the prefix behavior opam desires. I'd be happy to have a go at this as a PR if it sounds desirable.

[dbuenzli]

Related #6849

Note that to get proper support it would entail a bit more work than that e.g. so that packages are completed. But I'd also be happy to have cmdliner's completion in my shell rather than the current semi-broken opam completion scripts.