remove generator snippet interface

[davidjgoss]

🤔 What's changed?

Remove the "generator" snippet interface.

⚡️ What's your motivation?

We removed support for generators in steps some time ago.

🏷️ What kind of change is this?

• 💥 Breaking change (incompatible changes to the API)

📋 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: 98.561% (-0.002%) from 98.563% when pulling be2d14d on chore/snippets-breaking-changes into 260e943 on main.