chore: initialize standalone UPM package

Runtime/Core/IShrinkMod.cs

@@ -0,0 +1,10 @@
+namespace ShrinkModFramework
+{
+    public interface IShrinkMod
+    {
+        void OnConstruct(ShrinkModContext context);
+        void OnRegisterContent(ShrinkModContext context);
+        void OnInitialize(ShrinkModContext context);
+        void OnReady(ShrinkModContext context);
+    }
+}