enable autocompletion for spack-stack extensions

[AlexanderRichert-NOAA]

Description

This PR enables shell autocompletions for the spack-stack Spack extension commands. It modifies setup.sh to run spack commands --update-completion, which detects the argparse arguments defined in our extensions and translates them into shell completions. As far as I can tell this is the "right" way to implement this, and in particular it does not require any spack-stack-specific logic.

Dependencies

none

Issues addressed

none

Applications affected

n/a

Systems affected

all

Testing

• CI: Note whether the automatic tests (GitHub actions tests that run automatically for every commit) pass or not
  • GitHub actions CI tests pass
  • GitHub actions CI tests do not pass (provide explanation)
  • GitHub actions CI tests skipped (provide explanation if necessary)
• tested on personal computer

Checklist

• This PR addresses one issue/problem/enhancement or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.
• All necessary updates to the documentation on readthedocs are included in this PR
• All necessary updates to the spack-stack wiki will be made when this PR is merged

[climbfuji]

Thanks!