feat: make Unity integration optional

Integration/Runtime/ShrinkEventBus.Integration.Runtime.asmdef

@@ -0,0 +1,19 @@
+{
+  "name": "ShrinkEventBus.Integration.Runtime",
+  "rootNamespace": "ShrinkEventBus.Integration",
+  "references": [
+    "ShrinkEventBus.Runtime",
+    "ShrinkRuntime.Abstractions"
+  ],
+  "autoReferenced": true,
+  "defineConstraints": [
+    "SHRINK_EVENTBUS_HAS_SHARED_RUNTIME"
+  ],
+  "versionDefines": [
+    {
+      "name": "com.cneicy.shrink-runtime-abstractions",
+      "expression": "0.1.0",
+      "define": "SHRINK_EVENTBUS_HAS_SHARED_RUNTIME"
+    }
+  ]
+}

Integration/Runtime/ShrinkEventBus.Integration.Runtime.asmdef.meta

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+fileFormatVersion: 2
+guid: 86c48423db2e41e1b208c468296b7faf

Integration/Runtime/ShrinkSharedMainThreadDispatcher.cs

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+#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

Integration/Runtime/ShrinkSharedMainThreadDispatcher.cs.meta

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+fileFormatVersion: 2
+guid: f818313cb4e3447f9622b71decefa86c