[doc] add since version

Runtime/Internal/Attributes/SinceAttribute.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace XCharts.Runtime
+{
+    [AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
+    public class Since : Attribute
+    {
+        public readonly string version;
+
+        public Since(string version)
+        {
+            this.version = version;
+        }
+    }
+}

Runtime/Internal/Attributes/SinceAttribute.cs.meta

@@ -0,0 +1,11 @@
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