Refactor list syntax (dart-lang/markdown#499)

* Refactor ListSyntax

* Update CHANGELOG.md

* Some cleanup and fixing

* Add a test case for gitHubFlavored ExtensionSet

* Use `=>` syntax for `canParse`
24 files changed
tree: 0f6e293a2bb7b5cf6b7da22b2ebd9a223e9ac85c
  1. pkgs/