helper methods: iterate over selection spans in the document.

• Implement marks (superset of Selection/Range)
• Implement style configs, tab settings
• Visual tab width
• Refactor tree-sitter-highlight to work like the atom one, recomputing partial tree updates.
• Only render highlights on screen
• proper selection rendering
• Undo tree
• selection mode
• key sequence shortcuts (gg etc)

• syntax errors highlight query

• UI work: command line

• UI work: tab popup on command line
• UI work: completion popup
• UI work: floating pane