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/storybook/src/story_selector.rs

@@ -13,6 +13,7 @@ use ui::prelude::*;
 #[strum(serialize_all = "snake_case")]
 pub enum ComponentStory {
     AssistantChatMessage,
+    AssistantChatNotice,
     AutoHeightEditor,
     Avatar,
     Button,
@@ -46,6 +47,7 @@ impl ComponentStory {
             Self::AssistantChatMessage => {
                 cx.new_view(|_cx| assistant2::ui::ChatMessageStory).into()
             }
+            Self::AssistantChatNotice => cx.new_view(|_cx| assistant2::ui::ChatNoticeStory).into(),
             Self::AutoHeightEditor => AutoHeightEditorStory::new(cx).into(),
             Self::Avatar => cx.new_view(|_| ui::AvatarStory).into(),
             Self::Button => cx.new_view(|_| ui::ButtonStory).into(),