macos: use system beep for bell (#9339)

This seems pretty straightforward. I've tested it and it does what I'd
expect it to do.

Fixes #9338

macos/Sources/App/macOS/AppDelegate.swift

@@ -724,6 +724,10 @@ class AppDelegate: NSObject,
     }
 
     @objc private func ghosttyBellDidRing(_ notification: Notification) {
+        if (ghostty.config.bellFeatures.contains(.system)) {
+            NSSound.beep()
+        }
+
         if (ghostty.config.bellFeatures.contains(.attention)) {
             // Bounce the dock icon if we're not focused.
             NSApp.requestUserAttention(.informationalRequest)

src/config/Config.zig

@@ -2646,7 +2646,9 @@ keybind: Keybinds = .{},
 ///    This could result in an audiovisual effect, a notification, or something
 ///    else entirely. Changing these effects require altering system settings:
 ///    for instance under the "Sound > Alert Sound" setting in GNOME,
-///    or the "Accessibility > System Bell" settings in KDE Plasma. (GTK only)
+///    or the "Accessibility > System Bell" settings in KDE Plasma.
+///
+///    On macOS, this plays the system alert sound.
 ///
 ///  * `audio`
 ///