Build subtarget support for Unity 2021.2+ (#532)

* Subtarget support for Unity 2021.2+

Allows passing the -standaloneBuildSubtarget parameter via customParameters in order to facilitate dedicated server builds. Since Unity introduced the subtarget option in 2021.2, this parameter only takes effect in supported Unity versions.

* Fix build subtarget compatibility

Certain versions of Unity do not have a value for `NoSubtarget`. Using `default` will be more robust against churn in this area of Unity's API.

dist/default-build-script/Assets/Editor/UnityBuilderAction/Input/ArgumentsParser.cs

@@ -28,6 +28,7 @@ namespace UnityBuilderAction.Input
       }
 
       if (!Enum.IsDefined(typeof(BuildTarget), buildTarget)) {
+        Console.WriteLine($"{buildTarget} is not a defined {nameof(BuildTarget)}");
         EditorApplication.Exit(121);
       }