Provide source maps for pickles

[davidjgoss]

🤔 What's changed?

Change the code generation in the loader to construct an AST and include source maps back to the pickles.

⚡️ What's your motivation?

Fixes #145.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)
• ⚡ New feature (non-breaking change which adds new 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.

---

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

[davidjgoss]

• Get location directly from pickle once Add location to pickle messages#308 and Add location to pickle gherkin#433 are released