feat: make Unity integration optional

Integration/Runtime/ShrinkSharedMainThreadDispatcher.cs

@@ -0,0 +1,20 @@
+#if SHRINK_EVENTBUS_HAS_SHARED_RUNTIME
+using System;
+using ShrinkSDK.Runtime;
+
+namespace ShrinkEventBus.Integration
+{
+    public sealed class ShrinkSharedMainThreadDispatcher : IShrinkEventBusMainThreadDispatcher
+    {
+        private readonly IShrinkMainThreadDispatcher _dispatcher;
+
+        public ShrinkSharedMainThreadDispatcher(IShrinkMainThreadDispatcher dispatcher)
+        {
+            _dispatcher = dispatcher ?? throw new ArgumentNullException(nameof(dispatcher));
+        }
+
+        public bool IsMainThread => _dispatcher.IsMainThread;
+        public bool TryPost(Action action) => _dispatcher.TryPost(action);
+    }
+}
+#endif