Update iOS AppDelegate and SceneDelegate implementation

Refactor `AppDelegate` to conform to `FlutterImplicitEngineDelegate` and handle plugin registration via `didInitializeImplicitFlutterEngine`.
Update `SceneDelegate` to inherit from `FlutterSceneDelegate` and remove manual window management.

Author: CarGuo

ios/Runner/AppDelegate.swift

@@ -2,25 +2,17 @@ import UIKit
 import Flutter
 
 @main
-@objc class AppDelegate: FlutterAppDelegate {
+@objc class AppDelegate: FlutterAppDelegate, FlutterImplicitEngineDelegate {
+  func didInitializeImplicitFlutterEngine(
+    _ engineBridge: FlutterImplicitEngineBridge
+  ) {
+    GeneratedPluginRegistrant.register(with: engineBridge.pluginRegistry)
+  }
+
   override func application(
     _ application: UIApplication,
     didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?
   ) -> Bool {
-    GeneratedPluginRegistrant.register(with: self)
     return super.application(application, didFinishLaunchingWithOptions: launchOptions)
   }
-
-  override func application(
-    _ application: UIApplication,
-    configurationForConnecting connectingSceneSession: UISceneSession,
-    options: UIScene.ConnectionOptions
-  ) -> UISceneConfiguration {
-    let configuration = UISceneConfiguration(
-      name: "Default Configuration",
-      sessionRole: connectingSceneSession.role
-    )
-    configuration.delegateClass = SceneDelegate.self
-    return configuration
-  }
 }

ios/Runner/SceneDelegate.swift

@@ -1,5 +1,3 @@
-import UIKit
+import Flutter
 
-class SceneDelegate: UIResponder, UIWindowSceneDelegate {
-  var window: UIWindow?
-}
+class SceneDelegate: FlutterSceneDelegate {}