make typing for attach and log functions correct

[davidjgoss]

🤔 What's changed?

Make the typings for this.attach more accurate by providing overload definitions. Also correct the this.log type as it can never yield a promise.

Included tsd test that would fail without these changes.

⚡️ What's your motivation?

Fixes #2143 re the ambiguity of whether or not to expect a promise for stream attachments (depends on whether you pass in a callback), but it also improves the type safety around mediaType which is really only optional for string attachments.

🏷️ What kind of change is this?

• 🐛 Bug fix (non-breaking change which fixes a defect)

📋 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: 98.563%. Remained the same when pulling 437687b on fix/attachment-function-types into 93ee7ee on main.