• Preserve metadata passed to fenced code blocks as
  data-metadata on the created pre element.
• Update the README link to the markdown playground
  (https://dart-lang.github.io/tools).
• Update package:web API references in the example.
• Fix performance and correctness of HTML comment parser.
• Optimize indentation processing of fenced code blocks.
• Fix an issue with nested list structure when indented by tabs (#2172).
• Deprecated LinkContext and BlockParser.standardBlockSyntaxes.
  Implementation members that should never have been made public.
• Require Dart ^3.9.0.