Add workflow for Homebrew release automation

[nlydv]

As mentioned in #311 about automatically updating the Homebrew package on new release, this PR adds a workflow to do that. Just going to copy/paste the comment headers I wrote in the file:

# This workflow checks to see if the `kyokan-bob` package in
# Homebrew is updated to the latest release version. If it
# is not, then it will automatically open a PR to update it
# with the proper details in the Homebrew/homebrew-cask repo.

# Triggers on its own every time there's a new release, and
# no human input should thus be required to maintain it.
# Can also be run as a manual Action if necessary.

# If forking Homebrew/homebrew-cask is required to open the
# PR, a forked repo will get placed under the 'kyokan' org
# and get merged to Homebrew from there.

# A personal access token of an org member with write access,
# stored as repository secret named BARREL_ROLL, is required.
# The only scope needed for that token is `public_repo`.
# PR's generated by this workflow will be made by that user.

I also tested this properly by simulating the pipeline using my personal, non-official homebrew tap at nlydv/homebrew-brew containing a bobby.rb brew package, which sources from my fork at nlydv/bob-wallet, from which I published a dummy release of v0.999.2... which, after some more mundane steps... resulted in an automated PR that updates the bobby.rb file in nlydv/homebrew-brew as intended.

A screenshot of the workflow log:

(Notice, the resulting .dmg hash remained intact through the whole process and is equiv. to the hash of the official v0.9.0 release's .dmg file.)

[nlydv]

Changes in homebrew file to bump & support multi-arch dists got merged

Doesn’t effect this workflow though, it will update both Mac release assets in homebrew whenever a new public version gets released.

Rebased just to keep this PR fresh.