Update TabbedCommandBar DefaultStyleResourceUri to point to Toolkit vs. WinUI

See microsoft/microsoft-ui-xaml#3502

Also update TabbedCommandBarItem DataTemplate to use MUX version of NavigationViewItem

Author: michael-hawker

Microsoft.Toolkit.Uwp.UI.Controls.Core/TabbedCommandBar/TabbedCommandBar.cs

@@ -37,6 +37,7 @@ public class TabbedCommandBar : NavigationView
         public TabbedCommandBar()
         {
             DefaultStyleKey = typeof(TabbedCommandBar);
+            DefaultStyleResourceUri = new System.Uri("ms-appx:///Microsoft.Toolkit.Uwp.UI.Controls.Core/Themes/Generic.xaml");
 
             SelectionChanged += SelectedItemChanged;
         }