Restrict a Windows app to only be able to open one instance at a time.

1. Add WidgetsFlutterBinding.ensureInitialized(); to the start of your apps main function.
2. Add the async modifier to your apps main function.
3. Add a call to WindowsSingleInstance.ensureSingleInstance(), passing the apps startup args, a custom app string unique to your app (a-z, 0-9, _ and - only), and an optional callback function.

void main(List<String> args) async {
    WidgetsFlutterBinding.ensureInitialized();
    await WindowsSingleInstance.ensureSingleInstance(
        args,
        "custom_identifier",
        onSecondWindow: (args) {
            print(args);
        });
    runApp(const MyApp());
}