Introduce Tabs to Assistant Panel (#13783)

<img width="1652" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/8397ea87-51aa-4b69-85c7-bf963fe2b08a">

Release Notes:

- N/A

crates/assistant/src/model_selector.rs

@@ -79,6 +79,6 @@ impl RenderOnce for ModelSelector {
                         Tooltip::for_action("Change Model", &ToggleModelSelector, cx)
                     }),
             )
-            .anchor(gpui::AnchorCorner::BottomRight)
+            .attach(gpui::AnchorCorner::BottomLeft)
     }
 }