chore: use file-scoped namespace

src/Shared/HandyControl_Shared/Controls/Cover/CoverView/CoverViewItem.cs

@@ -1,7 +1,6 @@
-namespace HandyControl.Controls
+namespace HandyControl.Controls;
+
+public class CoverViewItem : HeaderedSelectableItem
 {
-    public class CoverViewItem : HeaderedSelectableItem
-    {
-        internal int Index { get; set; }
-    }
-}
+    internal int Index { get; set; }
+}