Provide an auto-publish GitHub action for Spago users

Today we provide a DailyImporter script which will attempt to publish any newly-discovered tags on existing registered repos from GitHub. We do this because there are packages — such as all of purescript as an organization — which haven't switched to Spago yet and therefore don't publish to the registry.

This has a number of issues:

• If a package version has a problem which prevents it from being published, the author has no idea and the version simply doesn't get registered.
• It only supports GitHub packages with version-like tags, which goes against the registry goals to be agnostic to where you store your code and to allow for monorepo/polyrepo setups

However, it's very convenient. So instead I propose we write and release a GitHub action which does the same thing the importer does — when you push a new SemVer tag it will auto-publish to the registry — that authors can install on their repositories.

It would produce the same effect but:

• the author is aware when there is an issue
• we could potentially make it flexible to detect polyrepo setups and use the subdir key to publish within a polyrepo without requiring a top level tag (could be triggered by something like contents of a commit message)

The daily import can coexist with this action easily as it doesn't republish already-published package versions. Even if there were a bug and it tried then the registry would reject the job.

The existing behavior is in scripts/src/DailyImporter.purs; start by reading that importer and the proposed SemVer-tag trigger. Done means a released GitHub Action that publishes tagged packages to the registry and makes failures visible to authors, while addressing the described repository layouts.