Automate gem build and deployment with github actions

[flavorjones]

I'd like to set up a github actions workflow with manual triggering to cut a new release of tailwindcss-rails.

We should use the trusted publishing capabilities of https://github.com/rubygems/release-gem

I chatted with @segiddins about this yesterday and have an idea of how to approach that I'd like to explore in https://github.com/flavorjones/ruby-c-extensions-explained first.

Also let's look at https://github.blog/2024-05-02-introducing-artifact-attestations-now-in-public-beta/

(Originally rails/tailwindcss-rails#362)