Add frontend subdirectory to project

frontend/macos/Flutter/GeneratedPluginRegistrant.swift

@@ -0,0 +1,12 @@
+//
+//  Generated file. Do not edit.
+//
+
+import FlutterMacOS
+import Foundation
+
+import shared_preferences_foundation
+
+func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
+  SharedPreferencesPlugin.register(with: registry.registrar(forPlugin: "SharedPreferencesPlugin"))
+}