chore: use file-scoped namespace

src/Shared/HandyControlDemo_Shared/Window/CustomNonClientAreaWindow.xaml.cs

@@ -1,10 +1,9 @@
-namespace HandyControlDemo.Window
+namespace HandyControlDemo.Window;
+
+public partial class CustomNonClientAreaWindow
 {
-    public partial class CustomNonClientAreaWindow
+    public CustomNonClientAreaWindow()
     {
-        public CustomNonClientAreaWindow()
-        {
-            InitializeComponent();
-        }
+        InitializeComponent();
     }
-}
+}