feat: make Unity integration optional

Runtime/IShrinkEventBusMainThreadDispatcher.cs

@@ -0,0 +1,12 @@
+#if UNITY_5_3_OR_NEWER
+using System;
+
+namespace ShrinkEventBus
+{
+    public interface IShrinkEventBusMainThreadDispatcher
+    {
+        bool IsMainThread { get; }
+        bool TryPost(Action action);
+    }
+}
+#endif