settings: Show error notification when settings are invalid (#15905)

https://github.com/user-attachments/assets/07627142-e730-4446-a50b-7ef46f8e661c

We want to improve the design in the future, but it fixes a long
standing paper cut for now.

Release Notes:

- Added a popup that is displayed when the settings are invalid

---------

Co-authored-by: Thorsten <thorsten@zed.dev>

crates/workspace/src/workspace.rs

@@ -5193,7 +5193,7 @@ fn activate_any_workspace_window(cx: &mut AsyncAppContext) -> Option<WindowHandl
     .flatten()
 }
 
-fn local_workspace_windows(cx: &AppContext) -> Vec<WindowHandle<Workspace>> {
+pub fn local_workspace_windows(cx: &AppContext) -> Vec<WindowHandle<Workspace>> {
     cx.windows()
         .into_iter()
         .filter_map(|window| window.downcast::<Workspace>())