chore: create a release pipeline

[jkowalleck]

goal: have a working, state-of-the-art pipeline for building and releasing this package to rubygems.

Requirements:

• tests run before the actual release, for QA reasons
• supports prereleases
• tags releases in git

optional:

• have releases on github with the build results as release artifacts.

[extern-c]

@jkowalleck

Looking at the issue again, it seems naturally split into two parts: RubyGems publishing (#59 (comment)) and GitHub Releases (optional). I can help with either if needed.