Switch to the system UI font on macOS (#10317)

To reference the system font, use the special ".SystemUIFont" family
name.

/cc @PixelJanitor 

Release Notes:

- Switched to the system UI  font for user interface elements on macOS.

Co-authored-by: Antonio Scandurra <antonio@zed.dev>

crates/gpui/src/platform/linux/text_system.rs

@@ -210,9 +210,16 @@ impl LinuxTextSystemState {
     #[profiling::function]
     fn load_family(
         &mut self,
-        name: &SharedString,
+        name: &str,
         _features: FontFeatures,
     ) -> Result<SmallVec<[FontId; 4]>> {
+        // TODO: Determine the proper system UI font.
+        let name = if name == ".SystemUIFont" {
+            "Zed Sans"
+        } else {
+            name
+        };
+
         let mut font_ids = SmallVec::new();
         let families = self
             .font_system