fix: ensure absolute paths (as file urls) can be loaded for formatters, snippets

[davidjgoss]

🤔 What's changed?

As part of the ESM support, we started loading formatters and snippet syntaxes with await import(). Unlike require(), paths you pass to import() have to be a valid URL, and for absolute paths that means using the file:// scheme.

This PR:

• Adds a test to prove that classes can be successfully loaded based on a file:// URL
• Fixes some code in the configuration module in order that file:// URLs are correctly accounted for when parsing the --format option (which unfortunately uses : as the delimiter)
• Adds a note to the documentation on formatters.

This will also be called out in the migration guide via #1962

⚡️ What's your motivation?

Fixes #1920.

🏷️ What kind of change is this?

• 🐛 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.

[coveralls]

Coverage increased (+0.002%) to 98.218% when pulling a1cbd47 on fix/formatter-file-urls into aa3972a on main.