Replace JUnit formatter with messages-based package

[davidjgoss]

🤔 What's changed?

This PR replaces the existing JUnit formatter - built using the old-style pattern - with the one from https://github.com/cucumber/junit-xml-formatter.

⚡️ What's your motivation?

The new formatter has parity with the Java implementation and uses the new plugin-based pattern with messages and query.

🏷️ What kind of change is this?

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

(This is not a breaking change because the old formatter was never publicly exported.)

📋 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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[coveralls]

coverage: 97.765% (-0.07%) from 97.838%
when pulling 0cc5c7b on junit-formatter-messages
into ddf424a on main.