Add Sign Out link for Supermaven (#14834)

Adds a menu item to sign out from a linked Supermaven account.


![image](https://github.com/user-attachments/assets/6af3c39f-9ca4-4ac2-a5c0-c7cb3c3aaac2)


Release Notes:

- Added the ability to sign out of a Supermaven account ([#12715(https://github.com/zed-industries/zed/issues/12715))

crates/supermaven/src/messages.rs

@@ -14,6 +14,7 @@ pub enum OutboundMessage {
     StateUpdate(StateUpdateMessage),
     #[allow(dead_code)]
     UseFreeVersion,
+    Logout,
 }
 
 #[derive(Debug, Serialize, Deserialize)]