standalone CLI shell completion support

[ethan-sargent]

Standalone CLI doesn't have a mechanism to produce shell completion information.

This makes the CLI less ergonomic to use/explore/test.

[pogilvie]

This is a great suggestion!

[crazynammer]

Check out this site @ethan-sargent. They did a plug-in help for me before for sfdx orchestrator - https://fig.io/

This was the issue. We can possibly leverage it.

[pogilvie]

The linked PR looks like it should work. It uses he same plugin that Salesforce uses I think

[ethan-sargent]

Thanks @pogilvie and @crazynammer.
Will check out fig.io - it looks interesting!

I've left the PR in draft for now because there's a bug with the installation instructions that get printed when running the command.

It's printing autocompleteundefinedzsh instead of autocomplete:zsh or autocomplete zsh, and not respecting the selected separator.

EDIT: note the command works fine if you substitute : for undefined, but it won't respect the space separation option which has more autocompletion features.

[azlam-abdulsalam]

@ethan-sargent can we merge this with a known issue?