Ruby: Introduce rubocop

[luke-hill]

🤔 What's changed?

Introduce rubocop - Fix a few simple AF cops

⚡️ What's your motivation?

Get full ruby suite code-clean

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing behaviour)

♻️ Anything particular you want feedback on?

This won't run in CI @davidjgoss. This is purely a dev only change as by the time we make this vaguely compliant we'll likely be moving the code over to the other repo

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

---

This text was originally generated from a template, then edited by hand. You can modify the template here.