Add Support for Shell Completion

[erikareads]

In order to support shell completion, we'll need something like a subcommand:

$ glint_cli __complete list of arguments h
completion1  description1
completion2  description2

Then we can use a version of cobras shell completion logic: https://github.com/spf13/cobra/blob/main/bash_completionsV2.go

I'm happy to do the PR that ports the completion logic for each shell from cobra, but glint will need to support the hidden subcommand to make that work.

[tankorsmash]

Having a way to dump the list of subcommands would be nice too; I was just looking to make a shallow FZF wrapper with my CLI but I can't find a way to list all the subcommands to pass to FZF. +1 on this issue.