Add basic CI testing

Some unit tests should get written, if the publishing of releases becomes automated as suggested in #8. The GitHub action for publishing a new version should only run, if the tests pass. The less code is covered, the higher the chances are for broken versions to be released.

Start by reading issue #8 and locating the GitHub Actions workflow that publishes releases, then inspect the repository's existing TypeScript test setup. Define a basic unit-test scope and connect the release workflow to the test result; done means the tests run in CI and a release cannot publish when they fail.

Written by the indexing model from the issue text.