[tui][edi] editor component upgrades

[nazmulidris]

1. Mimic github.com list handling (when pressing enter in a list or todo list item, it should automatically continue it)
2. Add "@done timestamp" when item is checked in smart list (also update md parser & syn hi to format this)
3. Add auto closing tags for `, ', ", [, (, etc. like most IDEs have
4. Add search, replace

Related issue: #100

• Add AST to the editor (similar to a DOM in a browser), so that it isn't lost after the file is parsed into a MdDocument.
• Keep this struct around, and provide a function to find the "element" at the caret.