chore: use file-scoped namespace

src/Shared/HandyControlDemo_Shared/Data/Model/TabControlDemoModel.cs

@@ -1,9 +1,8 @@
-namespace HandyControlDemo.Data
-{
-    public class TabControlDemoModel
-    {
-        public string Header { get; set; }
+namespace HandyControlDemo.Data;
 
-        public string BackgroundToken { get; set; }
-    }
-}
+public class TabControlDemoModel
+{
+    public string Header { get; set; }
+
+    public string BackgroundToken { get; set; }
+}