Add named export for CucumberHtmlStream

[davidjgoss]

🤔 What's changed?

Adds a named export CucumberHtmlStream, which was previously (and still is but deprecated) the default export.

⚡️ What's your motivation?

Currently the implementation of this formatter is split between here (the stream and template) and cucumber-js (forwarding messages, rewriting attachments). As we move to the new model with formatters as plugins, we'll shift the implementation left to here, and we'll want it to be the default export since that will be the standard for plugins. So, moving the stream to a separate named export now to give consumers a chance to switch and avoid a hard breaking change down the line.

🏷️ 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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