feat: add keyboard shortcuts for notes, the note list and the app

lib/shortcuts.dart

@@ -0,0 +1,16 @@
+import 'dart:ui';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter/widgets.dart';
+
+/// [key] with Cmd on Apple platforms and Ctrl elsewhere.
+SingleActivator primaryShortcut(LogicalKeyboardKey key, {bool shift = false}) {
+  final apple = defaultTargetPlatform == TargetPlatform.macOS ||
+      defaultTargetPlatform == TargetPlatform.iOS;
+  return SingleActivator(key, control: !apple, meta: apple, shift: shift);
+}
+
+final quitShortcut = primaryShortcut(LogicalKeyboardKey.keyQ);
+
+Future<void> quitApp() =>
+    ServicesBinding.instance.exitApplication(AppExitType.required);