Expose the AST in the JavaScript implementation

[aslakhellesoy]

🤔 What's changed?

The Cucumber Expression AST is exposed via CucumberExpression.ast. The AST types are exported.

⚡️ What's your motivation?

Having access to the AST is required to fix cucumber/language-service#16 and possibly also #51

🏷️ What kind of change is this?

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

♻️ Anything particular you want feedback on?

By doing this we're exposing the public API surface. If #41 ever gets implemented, it may require a change to the AST. We can live with that.

📋 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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