Set up pipeline to create plugin builds from the main branch

[nicholaskajoh]

Whenever a change is merged to main, run a pipeline to create a build of the plugin which can be downloaded and installed on a WordPress site. This will be the same pipeline as the one that runs tests on the main branch (see #2).

Steps:

• Run tests
• Remove non-production files (i.e test code, build config files etc)
• Zip plugin
• Create GitHub release and upload zipped plugin

Resources:

• https://stackoverflow.com/questions/75679683/how-can-i-auto-generate-a-release-note-and-create-a-release-using-github-actions
• https://michael-mckenna.com/how-to-upload-file-to-github-release-in-a-workflow/