feat: make Unity integration optional

Runtime/ShrinkEventBusRuntime.cs

@@ -2,25 +2,29 @@
 
 using System;
 using System.Threading;
-using ShrinkSDK.Runtime;
+#if UNITY_5_3_OR_NEWER
+using MainThreadDispatcherContract = ShrinkEventBus.IShrinkEventBusMainThreadDispatcher;
+#else
+using MainThreadDispatcherContract = ShrinkSDK.Runtime.IShrinkMainThreadDispatcher;
+#endif
 
 namespace ShrinkEventBus
 {
     public static class ShrinkEventBusRuntime
     {
-        private static IShrinkMainThreadDispatcher _mainThreadDispatcher = CreateDefaultDispatcher();
+        private static MainThreadDispatcherContract _mainThreadDispatcher = CreateDefaultDispatcher();
 
-        public static IShrinkMainThreadDispatcher MainThreadDispatcher =>
+        public static MainThreadDispatcherContract MainThreadDispatcher =>
             Volatile.Read(ref _mainThreadDispatcher);
 
-        public static void ConfigureMainThreadDispatcher(IShrinkMainThreadDispatcher dispatcher)
+        public static void ConfigureMainThreadDispatcher(MainThreadDispatcherContract dispatcher)
         {
             if (dispatcher == null)
                 throw new ArgumentNullException(nameof(dispatcher));
             Volatile.Write(ref _mainThreadDispatcher, dispatcher);
         }
 
-        private static IShrinkMainThreadDispatcher CreateDefaultDispatcher()
+        private static MainThreadDispatcherContract CreateDefaultDispatcher()
         {
 #if UNITY_5_3_OR_NEWER
             return new ShrinkUnityMainThreadDispatcher();
@@ -33,7 +37,7 @@ namespace ShrinkEventBus
     }
 
 #if UNITY_5_3_OR_NEWER
-    internal sealed class ShrinkUnityMainThreadDispatcher : IShrinkMainThreadDispatcher
+    internal sealed class ShrinkUnityMainThreadDispatcher : MainThreadDispatcherContract
     {
         public bool IsMainThread => Cysharp.Threading.Tasks.PlayerLoopHelper.IsMainThread;
 
@@ -48,7 +52,7 @@ namespace ShrinkEventBus
         }
     }
 #else
-    internal sealed class ShrinkSynchronizationContextDispatcher : IShrinkMainThreadDispatcher
+    internal sealed class ShrinkSynchronizationContextDispatcher : MainThreadDispatcherContract
     {
         private readonly SynchronizationContext? _context;
         private readonly int _threadId;