assistant2: Add temporary icon for the status bar item (#22156)

This is temporary and should be removed whenever we fully transition to
Assistant 2.

<img width="800" alt="Screenshot 2024-12-17 at 16 50 48"
src="https://github.com/user-attachments/assets/61137993-ec2c-4754-9f9a-7559d1b21d89"
/>

Release Notes:

- N/A

crates/ui/src/components/icon.rs

@@ -303,6 +303,7 @@ pub enum IconName {
     X,
     XCircle,
     ZedAssistant,
+    ZedAssistant2,
     ZedAssistantFilled,
     ZedPredict,
     ZedXCopilot,