macOS: implement cmd+[0-9] to goto tab

This is my attempt at fixing #63. It works! But:

1. The `NotificationCenter` subscription is triggered once for every
   open tab. That's obviously wrong. But I'm not sure and could use some
   pointers where else to put the subscription. That leads me to...
2. I'm _not_ knowledgable in Swift/AppKit/SwiftUI, so I might have put
   the wrong/right things in the wrong/right places. For example: wasn't
   sure what's to be handled in Swift and what's to be handled by the
   core in Zig.

Would love some pointers :)

src/apprt/embedded.zig

@@ -61,6 +61,9 @@ pub const App = struct {
 
         /// Focus the previous/next split (if any).
         focus_split: ?*const fn (SurfaceUD, input.SplitFocusDirection) callconv(.C) void = null,
+
+        /// Goto tab
+        goto_tab: ?*const fn (SurfaceUD, usize) callconv(.C) void = null,
     };
 
     core_app: *CoreApp,
@@ -359,6 +362,15 @@ pub const Surface = struct {
         };
     }
 
+    pub fn gotoTab(self: *Surface, n: usize) void {
+        const func = self.app.opts.goto_tab orelse {
+            log.info("runtime embedder does not goto_tab", .{});
+            return;
+        };
+
+        func(self.opts.userdata, n);
+    }
+
     /// The cursor position from the host directly is in screen coordinates but
     /// all our interface works in pixels.
     fn cursorPosToPixels(self: *const Surface, pos: apprt.CursorPos) !apprt.CursorPos {