Install rubocop rules + resolve issues + add CI check

[afomera]

Hey 👋 ,

We're hoping to setup Rubocop again on this repository and ensure we resolve issues and then install a CI check for the repo for this gem.

The rubocop rules can be found here:
https://github.com/hanakai-rb/repo-sync/blob/main/templates/gem/.rubocop.yml

Fixing the issues would be a result of running rubocop -a / rubocop -A and then hand fixing any that can't be auto-corrected.

Lastly:
When we've resolved all failures, we want to add a check to CI to ensure that our code stays passing.

the hanami/router gem provides a framework for how it's been done before: https://github.com/hanami/router/blob/main/script/ci
and the action will call that script.

If you need help or want to pick up this issue just give a comment!