support attachments with filenames

[davidjgoss]

🤔 What's changed?

Add support for providing an optional filename for attachments, which is emitted as part of the attachment message and thus can be used in formatters.

To achieve this, we make the second argument to this.attach an options object, with backwards compatibility so a media type string still works as before.

⚡️ What's your motivation?

Follow the cross-platform standards.

🏷️ What kind of change is this?

• ⚡ New feature (non-breaking change which adds new 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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[coveralls]

coverage: 98.505% (+0.006%) from 98.499% when pulling b1804d4 on feat/attachment-filenames into 0518bb3 on main.