assistant2: Add the ability to collapse chat messages (#11194)

This PR adds the ability to collapse/uncollapse chat messages.

I think the spacing might be a little off with the collapsed
calculations, so we'll need to look into that.


https://github.com/zed-industries/zed/assets/1486634/4009c831-b44e-4b30-85ed-0266cb5b8a26

Release Notes:

- N/A

Cargo.lock

@@ -379,6 +379,7 @@ dependencies = [
  "assets",
  "assistant_tooling",
  "client",
+ "collections",
  "editor",
  "env_logger",
  "feature_flags",