add workerId to TestCaseStarted

[davidjgoss]

🤔 What's changed?

Add an optional workerId field to TestCaseStarted message.

⚡️ What's your motivation?

See cucumber/cucumber-js#2085 (comment)

Cucumber implementations that support parallel running via worker processes or some distributed model can set workerId when a test case is started.

Cucumber itself won’t do anything with this (at least initially), but it will allow third-party reporting tools to do some useful reporting on the parallel run.

🏷️ What kind of change is this?

• ⚡ New feature (non-breaking change which adds new behaviour)

♻️ Anything particular you want feedback on?

📋 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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