switch to mocha for testing

[davidjgoss]

🤔 What's changed?

Remove Jest, and switch to a combination of Mocha, Chai and Sinon for testing, assertions and mocking respectively. React Testing Library is retained and works as before. Most of the diff is just changing the assertions to the Chai API, and some configuration.

For SCSS files, which used to be pretty much handled by Jest's module type mappings, I found the best compromise was to implement a small CSS loader (css-loader.mjs) to resolve to an object with keys for each class name found in the file.

Also pins to the latest 18.x Node.js version that supports register for module loaders, and ensures we use the current LTS for release jobs.

⚡️ What's your motivation?

In #338 we just about got Jest working with ESM, but it's super fragile - bringing in new dependencies for #337 immediately threw up some obscure errors.

Jest is also slow compared to Mocha, at least for our test suite.

🏷️ What kind of change is this?

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

📋 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.

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