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/item.rs

@@ -64,9 +64,9 @@ pub enum ClosePosition {
 #[derive(Copy, Clone, Debug, Default, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
 #[serde(rename_all = "snake_case")]
 pub enum ShowDiagnostics {
+    #[default]
     Off,
     Errors,
-    #[default]
     All,
 }
 
@@ -99,7 +99,7 @@ pub struct ItemSettingsContent {
     /// Which files containing diagnostic errors/warnings to mark in the tabs.
     /// This setting can take the following three values:
     ///
-    /// Default: all
+    /// Default: off
     show_diagnostics: Option<ShowDiagnostics>,
     /// Whether to always show the close button on tabs.
     ///

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),
             );