chore: initialize standalone UPM package

Runtime/Storage/IShrinkTutorialStorage.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Collections.Generic;
+
+namespace ShrinkTutorial
+{
+    [Serializable]
+    public sealed class ShrinkTutorialProgress
+    {
+        public List<string> completedTutorials = new();
+        public List<string> skippedTutorials = new();
+    }
+
+    public interface IShrinkTutorialStorage
+    {
+        ShrinkTutorialProgress Load();
+        void Save(ShrinkTutorialProgress progress);
+        void Reset();
+    }
+}