Setup github action for versioning and auto-publication when PR merge occurs

Not sure exactly what we should do/use for this.

I think ideally we have 2 primary branches in this repo: _master_ and _dev_.

When a PR is merged info dev, we publish a new snapshot version. When a PR is merged into master, we publish the new release stable version.

But given the generator project, we'll want to account for diverging versions between it and the modl plugin.

Will have to think on this some more.

Start by reviewing the repository's current branch structure and publication process, then compare the proposed dev and master merge flows. Clarify how snapshot and stable versions should be generated and how the generator project and modl plugin may diverge. Done means the versioning and publication design is agreed before implementing the GitHub Action.

Written by the indexing model from the issue text.