Default to built-in CSS and JS files

[davidjgoss]

🤔 What's changed?

The CucumberHtmlStream ctor now has default values for the CSS and JS file paths, so consumers can call the ctor without arguments and it will work as expected with the built-in CSS and JS.

⚡️ What's your motivation?

Fixes #342.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing 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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