feat: add cross-engine runtime support

ShrinkCommandEventBusBridge.cs

@@ -32,7 +32,9 @@ namespace ShrinkCommand.Integration
         private static readonly Dictionary<ShrinkCommandService, ServiceRegistration> RegisteredByService = new();
         private static IDisposable? _requestHandlerBinding;
 
+#if UNITY_5_3_OR_NEWER
         [UnityEngine.RuntimeInitializeOnLoadMethod(UnityEngine.RuntimeInitializeLoadType.SubsystemRegistration)]
+#endif
         private static void ResetStaticState()
         {
             lock (SyncRoot)