api: loadSources function

[davidjgoss]

🤔 What's changed?

• Add a loadSources function to the new API that does the Gherkin work and returns arrays of filtered, ordered pickles and parse errors
• Refactor runCucumber internals to use the same underpinning code
• Revert changes to parseGherkinMessageStream so there are no breaking changes there - it's now a standalone function that we can remove in a future major release (it's on the main entry point and is used by some third party code) once people have had time to switch over

⚡️ What's your motivation?

By popular demand via #1711.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, docuemntation 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.

[coveralls]

Coverage decreased (-0.06%) to 98.217% when pulling a808901 on feat/load-sources into 6feaefb on main.

[aurelien-reeves]

Beside the question, looks good 👌