use import by default for support code

[davidjgoss]

🤔 What's changed?

Previously if you specified neither require nor import options, Cucumber would look under your feature directories for .js files and load them using require(). As part of our gradual move to the newer JavaScript modules system, we want to have that default behaviour use await import() instead.

This should be backwards-compatible for at least 99% of cases, since CommonJS code can still be loaded with await import(). The require option should only be needed when transpiling with require-module - although in those cases users are almost certainly specifying the require option already due the different file extensions like .ts. There's now a warning emitted if require-module values are provided but no require which should help guide users. Nevertheless we'll release this under a major.

Also, update documentation to emphasise import over require, and JSON over other configuration file formats.

⚡️ What's your motivation?

From #2059:

The default behaviour (if neither of [require or import options] specified) uses await import()

🏷️ What kind of change is this?

• 💥 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.

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