Change the default staging and unstaging state display (#26299)

This adds a setting for the "border" hunk display mode, as discussed,
and makes it the default.

Here's how it looks in light mode:

<img width="1512" alt="Screenshot 2025-03-07 at 11 39 25 AM"
src="https://github.com/user-attachments/assets/a934faa3-ec69-47e1-ad46-535e48b98e9f"
/>

And dark mode: 

<img width="1511" alt="Screenshot 2025-03-07 at 11 39 56 AM"
src="https://github.com/user-attachments/assets/43c9afd1-22bb-4bd8-96ce-82702a6cbc80"
/>


Release Notes:

- Git Beta: Adjusted the default hunk styling for staged and unstaged
changes

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Nate <nate@zed.dev>

crates/editor/src/editor.rs

@@ -14882,14 +14882,14 @@ impl Editor {
         &self,
         window: &mut Window,
         cx: &mut App,
-    ) -> BTreeMap<DisplayRow, Background> {
+    ) -> BTreeMap<DisplayRow, LineHighlight> {
         let snapshot = self.snapshot(window, cx);
         let mut used_highlight_orders = HashMap::default();
         self.highlighted_rows
             .iter()
             .flat_map(|(_, highlighted_rows)| highlighted_rows.iter())
             .fold(
-                BTreeMap::<DisplayRow, Background>::new(),
+                BTreeMap::<DisplayRow, LineHighlight>::new(),
                 |mut unique_rows, highlight| {
                     let start = highlight.range.start.to_display_point(&snapshot);
                     let end = highlight.range.end.to_display_point(&snapshot);
@@ -18426,3 +18426,27 @@ impl Render for MissingEditPredictionKeybindingTooltip {
         })
     }
 }
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct LineHighlight {
+    pub background: Background,
+    pub border: Option<gpui::Hsla>,
+}
+
+impl From<Hsla> for LineHighlight {
+    fn from(hsla: Hsla) -> Self {
+        Self {
+            background: hsla.into(),
+            border: None,
+        }
+    }
+}
+
+impl From<Background> for LineHighlight {
+    fn from(background: Background) -> Self {
+        Self {
+            background,
+            border: None,
+        }
+    }
+}