Allow publishing via a github action

[oli-obk]

This action must be manually triggered.

It does multiple things:

• It checks whether the version bump follows semver via cargo-semver-checks.
• It tags the current commit with the version from the Cargo.toml.
• It publishes the version to crates.io via a token CARGO_REGISTRY_TOKEN that we'll need to put into this repo's secrets list.