Automatically install shell completions?

[trietsch]

@ariebovenberg shared an example for automatically installing shell completions with Homebrew. Let's update our Homebrew formula so that shell completions are installed automatically and we can remove the instructions from the caveats section.

[trietsch]

Stripe CLI seems to do this? https://github.com/stripe/stripe-cli

==> Installing stripe from stripe/stripe-cli
==> Downloading https://github.com/stripe/stripe-cli/releases/download/v1.7.4/stripe_1.7.4_mac-os_x86_64.tar.gz
==> Downloading from https://github-releases.githubusercontent.com/191848501/8ea5a989-5b3b-45df-b5a6-33a51550a4f5?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20211014%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date
######################################################################## 100.0%
==> /usr/local/Cellar/stripe/1.7.4/bin/stripe completion --shell bash
==> /usr/local/Cellar/stripe/1.7.4/bin/stripe completion --shell zsh
==> Caveats
❤ Thanks for installing the Stripe CLI! If this is your first time using the CLI, be sure to run `stripe login` first.

zsh completions and functions have been installed to:
  /usr/local/share/zsh/site-functions