GitHub Action for building a production-ready zip

[boonebgorges]

A number of build steps are required to make a version of the plugin that's runnable in WordPress. See the Release Checklist. https://github.com/PressForward/pressforward/wiki/Release-checklist

For the time being, I'd like to do the version bumps and readme updates manually, as they give me a way to do a last-minute review of changes.

But it would be nice if the GitHub Release that I create as the very last step actually had a usable zip attached. This would require a build step that runs the necessary npm and composer steps, then builds a zip that's compatible with WP's installation routine.

[boonebgorges]

This is done.