Markdown AST spec

Working draft to design lossless AST for markdown. Unstable.

Use issues tracker to discuss.

Goals & checkpoints:

1. Allow support use cases, not available with current implementations. Examples:
   • Optimize editor highlight update after text change.
   • When user selects range at html page, provide range in source markdown doc.
   • Better sync scroll for previews.
2. Provide source mapping info.
3. Allow AST -> markdown write without formatting loss.
4. Performance:
   • Additional care should be applied to fast AST build/traverse in Javascript.
5. Analyse required operation and recommend minimal set of API functions.

Useful to read:

• csstree - interesting approach to fast tokenizer and other optimizations.
• ESTree - lossless JS AST design.
• AST Explorer - explore different AST implementations.