Implement GitHub action to lint JS

#51 and #60 implemented initial JS functionality (not in production, but for plugin maintenance tooling). This includes basic ES linting and formatting relying on @wordpress/scripts, which is what we should rely on here to have an easy-to-maintain setup. What's still missing is a GitHub workflow js-lint.yml that performs the linting check on commits and pull-requests.

See the existing php-lint.yml workflow for when exactly this new workflow should run.