chore: initialize standalone UPM package

Runtime/Core/ShrinkModBase.cs

@@ -0,0 +1,10 @@
+namespace ShrinkModFramework
+{
+    public abstract class ShrinkModBase : IShrinkMod
+    {
+        public virtual void OnConstruct(ShrinkModContext context) { }
+        public virtual void OnRegisterContent(ShrinkModContext context) { }
+        public virtual void OnInitialize(ShrinkModContext context) { }
+        public virtual void OnReady(ShrinkModContext context) { }
+    }
+}