Add Rubocop, assume Ruby 3.1.1

[jcraigk]

This adds Rubocop and assumes Ruby 3.1.1 which is the latest stable Ruby as of this writing.

https://github.com/rubocop/rubocop

Before I start chipping away at the non-auto-correctable items listed in the.rubocop_todo.yml file, I thought I would get some feedback on this and make sure this is a welcome change. The .rubocop.yml contains some defaults I like, but is easy to change based on the specifics of the project and the preferences of the maintainers.

[acook]

Hi @jcraigk, thanks for your contributions! I really appreciate it.

I've been buried lately and somehow missed all my notifications, so apologies for the delay getting back to you.

I really like Rubocop as a concept, I've used it at pretty much every place of employment on the basis of their internal code style. For my personal projects I have a particular style I use and never got around to changing all the settings and setting it up on my projects.

A lot of the defaults go against the best practices that I have developed over the years, and there tends to be good-ish reasons for why I do them this way in my personal projects.

BUT some of my practices are out of date due to the age of some of the original code in this repo, I'll get back to you soon with my thoughts on this!

[acook]

I updated the Ruby version to the latest and put my thoughts on Rubocop's defaults here if you're interested: https://gist.github.com/acook/5aa0aab483b3b8e9cb385184b71a8518