Added async implementations to the HtmlFormatter.

[clrudolphi]

🤔 What's changed?

Added async method implementations.

⚡️ What's your motivation?

Reqnroll will want to consume the async implementations in its implementation of Messages.

🏷️ What kind of change is this?

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

♻️ Anything particular you want feedback on?

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• [X ] I've changed the behaviour of the code
  • [ X] 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
  • [ X] I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[gasparnagy]

Do we need to maintain both the async and the sync methods? Would it be just enough to keep the async one?

[gasparnagy]

Based on the warning, this method should have been done anyway.

'JsonInHtmlWriter.WriteAsync(char[])' hides inherited member 'TextWriter.WriteAsync(char[])'. Use the new keyword if hiding was intended.

[mpkorstanje]

Do we need to maintain both the async and the sync methods?

Your call. The only difference is the resulting version number. If sync is removed then it will be a major release, otherwise a minor.

[clrudolphi]

Do we need to maintain both the async and the sync methods? Would it be just enough to keep the async one?

I put the [Obsolete] marker on the sync constructors.
We can release this as a minor and come back later to remove them completely.