Switch text content representation to rope

[Nimaoth]

Current representation is an array of lines.
Finish Sum tree and rope implementations and integrate into the editor.

Advantages:

• Better performance for large files
• Immutable snapshots of buffer content which can be shared between threads
  • Saving, treesitter parsing/reparsing, etc can be done in background thread without blocking UI
• Easier sharing of buffer content without having to pass around the entire TextDocument

[Nimaoth]

Done by #26