esm: use specific option for import vs require for support code

[davidjgoss]

🤔 Motivation and changes

As part of the initial work to support ESM, I made it so that all support code was imported by default, except sometimes --- based on heuristics around file extension and whether --require-module had been specified --- it fell back to require. It was a kind of aggressive move towards the new module system.

On reflection I think this was a mistake. The heuristics are bound to be wrong some of the time and potentially leave users with a breaking change that's difficult to work around. A future major release where we go ESM-only is probably not far away and so it makes sense to drop require entirely at that point.

Fortunately the ESM support has only gone as far as the 8.x RC so it's not too late to pivot. This PR makes it so that:

• --require does the same as it always did before, no breaking changes
• --import, a new option, works like --require but imports instead, so we'd expect ESM projects taking advantage of the new support in 8.x to use this
• If neither --require or --import are specified, the automatic requireing does what it did before based on feature paths, but also any .mjs files found in the same way are imported - this is a backwards-compatible change because .mjs files didn't work at all previously

Not rowing back on formatters and snippets using import - I think this still right because it's much more predictable and there's no just-in-time transpilation involved to complicate things.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt (improvement to code design or tooling without changing behaviour)
• 🐛 Bug fix (non-breaking change which fixes a defect)

📋 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]

Still need to add and update some docs but feedback on the what and why would be great.

[coveralls]

Coverage increased (+0.05%) to 98.21% when pulling a789e3b on import-own-config-option into 0195962 on main.

[aurelien-reeves]

Looks good

[davidjgoss]

No objections so far; I'll update the documentation tonight and merge tomorrow.