api: clean up, add docs, add entry point

[davidjgoss]

🤔 What's changed?

• Clean up some details of the new API, including making arguments optional where possible
• Add API Extractor to analyse and generate docs
  • Tweak some code to make this come out better - consistent naming of types/interfaces, split out some named interfaces for things that were just inline before, etc - e.g. IRunEnvironment and mergeEnvironment is all about that
  • Eventually I'd like to also document the main entry point with this, but that's a huge piece of work and it seems like a good idea to trial it on a smaller scale here
• Add an entry point so you can require or import from @cucumber/cucumber/api - keen to avoid adding yet more stuff to the main entry point, which should be kept for normal end user functions

⚡️ What's your motivation?

#1711

🏷️ What kind of change is this?

• ⚡ 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.

[davidjgoss]

When reviewing, bear in mind the following things are generated:

• reports/cucumber.api.md
• docs/api/**

[coveralls]

Coverage increased (+0.006%) to 98.272% when pulling 24b420d on feat/api-entry-point-and-docs into cc633c5 on main.

[aurelien-reeves]

I did not review everything yet
I still have a few comments. No big deal :)

[aurelien-reeves]

Looks pretty good.

I did not review the whole API documentation, still I approve how it is generated and the improvements regarding the API itself