Remove Runtime and other deprecated exports

[davidjgoss]

🤔 What's changed?

Removes some long-deprecated exports from the main entry point:

• Runtime
• parseGherkinMessageStream
• PickleFilter

⚡️ What's your motivation?

The time has come for all of these, but Runtime in particular is the subject of a rewrite in #2419.

🏷️ What kind of change is this?

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

---

This text was originally generated from a template, then edited by hand. You can modify the template here.

[coveralls]

coverage: 97.929% (-0.02%) from 97.946%
when pulling dd90b76 on feat/remove-runtime
into dc03f16 on main.