assistant2: Add ChatNotice component (#11262)

This PR adds a new `ChatNotice` component for displaying notices within
the assistant.

We will be able to use this for allowing the user to confirm a project
index request.

<img width="518" alt="Screenshot 2024-05-01 at 2 41 31 PM"
src="https://github.com/zed-industries/zed/assets/1486634/3beaeeaf-6046-4284-ac0a-4248e1f9ac20">

Release Notes:

- N/A

crates/assistant2/src/ui/chat_message.rs

@@ -118,7 +118,7 @@ impl RenderOnce for ChatMessageHeader {
                 h_flex()
                     .gap_3()
                     .map(|this| {
-                        let avatar_size = rems(20.0 / 16.0);
+                        let avatar_size = rems_from_px(20.);
                         if let Some(avatar_uri) = avatar_uri {
                             this.child(Avatar::new(avatar_uri).size(avatar_size))
                         } else {