Set up a CI script to automatically create a production-ready package

[GabrielMajeri]

Instead of having to run composer install each time I want to (re)install this plugin in production, I'd much rather see we have some CI script which builds a packaged-up version of the plugin, with all the dependencies installed, needing only to be extracted into an osTicket install.

This could be done automatically using GitHub Actions, and the final .zip file could be published as a release in this repo.