Separate execution from test declaration

[davidjgoss]

🤔 What's changed?

Refactor the runner implementation to bring more of the orchestration login into internal code and keep the interactions with the Node.js test runner (suite and test) as unpolluted as possible. Push these into the code that the loader generates.

⚡️ What's your motivation?

Part of #145

The next step is to include source maps in the generated code so locations cited in tests point back to the pickle locations in the feature file.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing 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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