✨ Introduce AlertBlockSyntax (dart-lang/markdown#570)

* ✨ Introduce `CalloutBlockSyntax`

* ⚡️ More flex patterns

* 🚀 Produce correct structure

* ⚡️ Case-insensitive types

* ⚡️ Improve `canParse`

* ✅ Fill test cases

* Update AUTHORS

* 🚚 Rename

* 📝 Add CHANGELOG

* 🔥 Remove `zh`

* Update CHANGELOG.md

* Update pubspec.yaml

* 🔖 Fix version

* 🧪 Add escape brackets case

* ⚡️ const type text map