Rich text API

[bartfeenstra]

• document-based (this would actually be a separate follow-up feature)
  • titles
  • dates
  • authors
• per-text content type, e.g. any piece of natural language has a content type and a locale
• allow plain text extraction (for search index)
• Implement this for Note and Described
• Introduce a new RichText interface to capture this
• Update existing methods to return not just str but str | RichTextStr where RichTextStr(RichText, str)
• Can this leverage renders somehow? Maybe those should provide extensions and content types they can render. Or better, just content types and keep a global content type map. Does a library for that already exist?

This can be used to represent pages with managed content, but also notes attached to entities