Adds static instance getter to DefaultMobileMenuWidgetBuilder (#335)

Calling `DeviceInfoPlugin().deviceInfo` for every instance of
`ContextMenuWidget` is expensive if there are multiple ones created
during single frame. This adds a static instance accessor to
`DefaultMobileMenuWidgetBuilder` to reuse result of a single call to
`DeviceInfoPlugin().deviceInfo`.

super_context_menu/lib/src/default_builder/mobile_menu_widget_builder.dart

@@ -197,6 +197,9 @@ class DefaultMobileMenuWidgetBuilder extends MobileMenuWidgetBuilder {
     }
   }
 
+  static final DefaultMobileMenuWidgetBuilder instance =
+      DefaultMobileMenuWidgetBuilder();
+
   /// Allows overriding brightness for the menu UI.
   final Brightness? _brightness;
 

super_context_menu/lib/src/menu.dart

@@ -65,7 +65,7 @@ class ContextMenuWidget extends StatelessWidget {
   })  : assert(previewBuilder == null || deferredPreviewBuilder == null,
             'Cannot use both previewBuilder and deferredPreviewBuilder'),
         mobileMenuWidgetBuilder =
-            mobileMenuWidgetBuilder ?? DefaultMobileMenuWidgetBuilder(),
+            mobileMenuWidgetBuilder ?? DefaultMobileMenuWidgetBuilder.instance,
         desktopMenuWidgetBuilder =
             desktopMenuWidgetBuilder ?? DefaultDesktopMenuWidgetBuilder();