Replace rich_text with markdown in assistant2 (#11650)

We don't implement copy yet but it should be pretty straightforward to
add.


https://github.com/zed-industries/zed/assets/482957/6b4d7c34-de6b-4b07-aed9-608c771bbbdb

/cc: @rgbkrk @maxbrunsfeld @maxdeviant 

Release Notes:

- N/A

crates/assistant2/src/ui/chat_message.rs

@@ -119,7 +119,7 @@ impl RenderOnce for ChatMessage {
             )
             .when(self.messages.len() > 0, |el| {
                 el.child(
-                    h_flex().child(
+                    h_flex().w_full().child(
                         v_flex()
                             .relative()
                             .overflow_hidden()