This should finally allow us to adopt code style lints, as well
as others such as performance linting, and have them enforced by
the Github Actions continuous integration (CI) jobs.
For now, I'm choosing to depend on the 'panolint' gem from
Panorama Education, which is my current workplace, as I'd like
to adopt and stay consistent with the setting used there.
Changes
* Move development and test dependencies from gemspec to Gemfile
* Add dependency on 'panolint' gem
* Add .rubocop.yml
* Fix all existing issues
* Run rubocop in github actions CI
Marc Siegel