assistant2: Setup storybook (#11228)

This PR sets up the `assistant2` crate with the storybook so that UI
elements can be iterated on in isolation.

To start, we have some stories for the `ChatMessage` component:

```sh
cargo run -p storybook -- components/assistant_chat_message
```

<img width="1233" alt="Screenshot 2024-04-30 at 5 20 03 PM"
src="https://github.com/zed-industries/zed/assets/1486634/510967ea-0e9b-4fa9-94fb-421ee74bcc45">

Release Notes:

- N/A

crates/assistant2/src/ui.rs

@@ -1,5 +1,11 @@
 mod chat_message;
 mod composer;
 
+#[cfg(feature = "stories")]
+mod stories;
+
 pub use chat_message::*;
 pub use composer::*;
+
+#[cfg(feature = "stories")]
+pub use stories::*;