Refactor HTML block syntax (dart-lang/markdown#458)

* Refactor HTML blocks syntax

* Update a comment

* Update some comments

* Put some files back and add deprecation annotation

* Update CHANGELOG.md

* Fix some comments

* Update a comment

* Export HtmlBlockSyntax

* Expand some comments
16 files changed
tree: 14da2b9b13bb5092a9a81d6c123e98383c4b4383
  1. pkgs/