Don't set font-size, use ems consistently

[davidjgoss]

🤔 What's changed?

Previously, the library included a font-size: 14px that was overly opinionated and resulted in smaller type than generally looks good in most contexts. This PR removes that setting and ensures that font-size declarations elsewhere are consistently in em units. This means we'll inherit the nearest declared parent font-size, or otherwise just the root font-size which is 16px by default. Also, headings for Gherkin keywords have been adjusted so they work well at that default size.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. 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.

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