Add formatter plugins and start reimplementing builtins

[davidjgoss]

🤔 What's changed?

• Add a new "formatter" kind of plugin (still internal)
• Reimplement a couple of builtin formatters (html and message) as plugins and ensure they still work

The formatter plugin has a very lean interface for now - it can listen for messages, write to the output, and access the formatOptions from the configuration. The idea is to reimplement more builtin formatters and in doing so find and fill limitations, and probably also build out a useful test harness or pattern for people writing their own formatters.

So far we have only changes formatters that are not exported, to avoid any breaking changes and give us room to alter course if we need to in the short term. Next candidates for reimplementing will be:

• junit (recently added, not exported)
• json (because of make json formatter reusable outside cucumber-js #2239, will still export legacy version for a compatibility period)
• progress-bar (not exported, needs direct access to the stream)
• @cucumber/pretty-formatter (separate package, needs access to snippet syntax)

⚡️ What's your motivation?

Per #2091 we're working towards a plugin system, and that includes bring formatters into that with a leaner message-based interface.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)
• ⚡ 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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[coveralls]

coverage: 98.233% (-0.2%) from 98.402%
when pulling 8ff54d6 on feat/new-formatter-part-1
into 6dae044 on main.

[davidjgoss]

Cleaning up formatter streams should happen after formatter plugins have had a chance to do their own cleaning up.