Refine diagnostic icons in tabs (#21637)

Follow up to https://github.com/zed-industries/zed/pull/21383

Mostly adjusting the alignment when there are no file icons.

<img width="800" alt="Screenshot 2024-12-06 at 08 35 48"
src="https://github.com/user-attachments/assets/6a4206cc-2af5-4317-a92e-49dffa37de99">

Release Notes:

- N/A

crates/workspace/src/pane.rs

@@ -2145,7 +2145,7 @@ impl Pane {
                     .child(if let Some(decorated_icon) = decorated_icon {
                         div().child(decorated_icon.into_any_element())
                     } else if let Some(icon) = icon {
-                        div().child(icon.into_any_element())
+                        div().mt(px(2.5)).child(icon.into_any_element())
                     } else {
                         div()
                     })