Adjust diagnostic in tabs behavior (#21671)

Follow up to https://github.com/zed-industries/zed/pull/21637

After discussing about this feature with the team, we've decided that
diagnostic display in tabs should be: 1) turned off by default, and 2)
only shown when there are file icons. The main reason here being to keep
Zed's UI uncluttered.

This means that you can technically have this setting:

```
  "tabs": {
    "show_diagnostics": "all"
  },
```

...and still don't see any diagnostics because you're missing
`file_icons": true`.

| Error with file icons | Error with no file icons |
|--------|--------|
| <img width="800" alt="Screenshot 2024-12-06 at 21 05 13"
src="https://github.com/user-attachments/assets/babf9cc3-b3b0-492e-9748-3e97d96ce90e">
| <img width="800" alt="Screenshot 2024-12-06 at 21 05 24"
src="https://github.com/user-attachments/assets/5247a5f1-55a0-4c56-8aaf-a0cdd115464f">
|


Release Notes:

- N/A

crates/workspace/src/pane.rs

@@ -2002,12 +2002,8 @@ impl Pane {
 
         let icon = if decorated_icon.is_none() {
             match item_diagnostic {
-                Some(&DiagnosticSeverity::ERROR) => {
-                    Some(Icon::new(IconName::X).color(Color::Error))
-                }
-                Some(&DiagnosticSeverity::WARNING) => {
-                    Some(Icon::new(IconName::Triangle).color(Color::Warning))
-                }
+                Some(&DiagnosticSeverity::ERROR) => None,
+                Some(&DiagnosticSeverity::WARNING) => None,
                 _ => item.tab_icon(cx).map(|icon| icon.color(Color::Muted)),
             }
             .map(|icon| icon.size(IconSize::Small))
@@ -2144,13 +2140,17 @@ impl Pane {
             .child(
                 h_flex()
                     .gap_1()
-                    .child(if let Some(decorated_icon) = decorated_icon {
-                        div().child(decorated_icon.into_any_element())
-                    } else if let Some(icon) = icon {
-                        div().mt(px(2.5)).child(icon.into_any_element())
-                    } else {
-                        div()
-                    })
+                    .items_center()
+                    .children(
+                        std::iter::once(if let Some(decorated_icon) = decorated_icon {
+                            Some(div().child(decorated_icon.into_any_element()))
+                        } else if let Some(icon) = icon {
+                            Some(div().child(icon.into_any_element()))
+                        } else {
+                            None
+                        })
+                        .flatten(),
+                    )
                     .child(label),
             );