chore: initialize standalone UPM package

Runtime/Metadata/ShrinkModState.cs

@@ -0,0 +1,12 @@
+namespace ShrinkModFramework
+{
+    public enum ShrinkModState
+    {
+        Discovered = 0,
+        Constructed = 1,
+        ContentRegistered = 2,
+        Initialized = 3,
+        Ready = 4,
+        Unloaded = 5
+    }
+}