Make `transformer` of defineParameterType optional

[badeball]

🤔 What's changed?

Make transformer of defineParameterType optional.

⚡️ What's your motivation?

This is to accurately reflect actual behavior, as per documentation and tests.

🏷️ What kind of change is this?

Minor, backward compatibility type-change.

♻️ Anything particular you want feedback on?

Nah, nothing aside from the change itself.

📋 Checklist:

• [x] I agree to respect and uphold the Cucumber Community Code of Conduct
• [x] I've changed the behaviour of the code
  • [ ] I have added/updated tests to cover my changes.
  • This is already tested.
• [ ] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.
• [x] Users should know about my change
  • [x] 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.

[coveralls]

coverage: 98.397%. remained the same when pulling c11f4e21ef57ea7d389d27f26717497196c8774e on optional-transformer into f18a0045a584f0597b8af83d323e74d6f34331e3 on main.

[davidjgoss]

Released in https://github.com/cucumber/cucumber-js/releases/tag/v10.5.1