notifications: add XNotifications + macOS UNUserNotificationCenter plugin

Generic OS-notification + window-focus surface. Hand-rolled MethodChannel
+ EventChannel, registered through XPlugin alongside the existing camera
/ video / clipboard plugins. macOS native handler uses
UNUserNotificationCenter with thread-grouping support, NSApp activation
on tap, and NSApplicationDidBecomeActive/DidResignActive for the focus
signal (more reliable than Flutter's AppLifecycleState on macOS).

macos/Classes/XPlugin.swift

@@ -12,6 +12,7 @@ public class XPlugin: NSObject, FlutterPlugin {
             CameraPlugin(),
             UxVideoPlayerPlugin(),
             UrlPlugin(),
+            NotificationsPlugin(),
         ]
         for plugin in plugins {
             plugin.register(with: registrar)