Default to Zed Sans for UI (#4045)

Release Notes:

- Improve legibility of default UI font

assets/settings/default.json

@@ -36,7 +36,7 @@
   //         },
   "buffer_line_height": "comfortable",
   // The name of a font to use for rendering text in the UI
-  "ui_font_family": "Zed Mono",
+  "ui_font_family": "Zed Sans",
   // The OpenType features to enable for text in the UI
   "ui_font_features": {
     // Disable ligatures:

crates/gpui/src/text_system.rs

@@ -59,7 +59,7 @@ impl TextSystem {
             fallback_font_stack: smallvec![
                 // TODO: This is currently Zed-specific.
                 // We should allow GPUI users to provide their own fallback font stack.
-                font("Zed Mono"),
+                font("Zed Sans"),
                 font("Helvetica")
             ],
         }