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/inline_completion_button/src/inline_completion_button.rs

@@ -298,7 +298,9 @@ impl InlineCompletionButton {
 
     fn build_supermaven_context_menu(&self, cx: &mut ViewContext<Self>) -> View<ContextMenu> {
         ContextMenu::build(cx, |menu, cx| {
-            self.build_language_settings_menu(menu, cx).separator()
+            self.build_language_settings_menu(menu, cx)
+                .separator()
+                .action("Sign Out", supermaven::SignOut.boxed_clone())
         })
     }