chore: Add App Store screenshots

Refs:

test_driver/integration_test.dart

@@ -0,0 +1,19 @@
+import 'dart:io';
+
+import 'package:flutter_driver/flutter_driver.dart';
+import 'package:integration_test/integration_test_driver_extended.dart';
+
+Future<void> main() async {
+  final driver = await FlutterDriver.connect();
+  await integrationDriver(
+    driver: driver,
+    onScreenshot: (name, image, [args]) async {
+      final outputDir = Platform.environment['SCREENSHOT_OUTPUT_DIR'] ??
+          'ios/fastlane/screenshots/en-US';
+      final file = File('$outputDir/$name.png');
+      await file.parent.create(recursive: true);
+      await file.writeAsBytes(image);
+      return true;
+    },
+  );
+}