CmdPal: Fix SUI crash ; Allow extensions to be disabled (#38040)

Both `TopLevelCommandItemWrapper` and `TopLevelViewModel` were really the same thing. The latter was from an earlier prototype, and the former is a more correct, safer abstraction. We really should have only ever used the former, but alas, we only used it for the SUI, and it piggy-backed off the latter, and that meant the latter's bugs became the former's.


tldr: I made the icon access safe in the SUI. 

And while I was doing this, because we now have a cleaner VM abstraction here in the host, we can actually cleanly disable extensions, because the `CommandProviderWrapper` knows which `ViewModel`s it made. 

Closes https://github.com/zadjii-msft/PowerToys/issues/426
Closes https://github.com/zadjii-msft/PowerToys/issues/478
Closes https://github.com/zadjii-msft/PowerToys/issues/577

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Properties/Resources.Designer.cs

@@ -303,6 +303,15 @@ namespace Microsoft.CmdPal.UI.ViewModels.Properties {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Disabled.
+        /// </summary>
+        public static string builtin_disabled_extension {
+            get {
+                return ResourceManager.GetString("builtin_disabled_extension", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Built-in commands.
         /// </summary>

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Properties/Resources.resx

@@ -236,4 +236,7 @@
   <data name="builtin_reload_display_title" xml:space="preserve">
     <value>Reload Command Palette extensions</value>
   </data>
+  <data name="builtin_disabled_extension" xml:space="preserve">
+    <value>Disabled</value>
+  </data>
 </root>