better formats config

[davidjgoss]

🤔 What's changed?

Expressing formats like "html:./report.html" is painful to read and write, and difficult for us to process. It gets worse when you want to include things that have colons, like URLs and Windows absolute file paths, some of which we struggle to support smartly. It's unavoidable on the CLI but in a configuration file we can make it easier.

This PR adds support for a two-item array notation for format items, so for the above example you could instead do ["html", "./report.html"].

🏷️ 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.

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