Add Rubocop

[TydaJL]

RuboCop is a Ruby code style checker (linter) and formatter.

• Add 'rubocop' to your Gemfile (Instructions: https://docs.rubocop.org/rubocop/installation.html)
• Add any dependent Gems for Rubocop
• Add this Workflow to your Workflows (Github Actions): https://github.com/cs169/BJC-Teacher-Tracker/blob/main/.github/workflows/rubocop.yml
• Add this Config file as a hidden file (file name starting with a ".") in your project root folder: https://github.com/cs169/BJC-Teacher-Tracker/blob/main/.rubocop.yml
• Add ‘Rubocop’ badge to README.md after passing.

Teams can run rubocop -a to make ‘safe’ fixes and rubocop -A to make unsafe auto-refactors.
You can also run rubocop --auto-gen-config to configure some finer settings.

You can use/create the :linters group in Gemfile to separate rubocop from the other gems.