assistant2: Sketch in chat editor (#21116)

This PR sketches in the chat editor for `assistant2`.

<img width="1136" alt="Screenshot 2024-11-23 at 1 56 23 PM"
src="https://github.com/user-attachments/assets/6e979995-c0cf-4a46-8694-fc4a8646336f">

Release Notes:

- N/A

Cargo.lock

@@ -456,11 +456,14 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "command_palette_hooks",
+ "editor",
  "feature_flags",
  "gpui",
  "language_model",
  "language_model_selector",
  "proto",
+ "settings",
+ "theme",
  "ui",
  "workspace",
 ]