chore: initialize standalone UPM package

Runtime/IShrinkEventBus.cs

@@ -0,0 +1,23 @@
+#nullable enable
+
+using System;
+using System.Threading;
+using Cysharp.Threading.Tasks;
+
+namespace ShrinkEventBus
+{
+    public interface IShrinkEventBus : IDisposable
+    {
+        ShrinkBusKey Key { get; }
+        ShrinkBusOptions Options { get; }
+
+        ShrinkPostResult Post<TEvent>(in TEvent eventData)
+            where TEvent : IShrinkEvent;
+
+        UniTask<ShrinkPostResult> PostAsync<TEvent>(TEvent eventData,
+            CancellationToken cancellationToken = default)
+            where TEvent : IShrinkEvent;
+
+        IDisposable Attach(object target);
+    }
+}