support prereleases

**Is your feature request related to a problem? Please describe.**
GitHub releases can be marked as prerelease. It would make sense to do this for release candidates.

**Describe the solution you'd like**
An optional `prerelease` input that defaults to `false` which we could trigger from our release action in consuming projects.

**Describe alternatives you've considered**
Manually updating releases once they're created, as I've done [with cucumber-js](https://github.com/cucumber/cucumber-js/releases).

**Additional context**
Not a high priority - we're doing it when publishing to npm which is important as it affects user installs, GitHub is more of a reference point.