Update major version ref after release is published

[richtea]

When a release is published, it creates a tag with a 3-level semantic version, e.g. v1.2.3. But the ref most commonly in use to specify the GH action version is the major ref, e.g. v1 (see the GH documentation for more details).

We should add a new workflow that handles a release published event, and automatically moves (or creates) the major version ref as required. The ref shouldn't be moved or created if the release is a beta or other prerelease version.

Although the docs specify using a tag as the ref, we should consider using a branch instead, as discussed in actions/toolkit#214