Standardizing built-in extension icon handling (#40606)

Just standardizing built-in extensions to use a `internal sealed class
Icons` for all their non-dynamic icons.

Looks like a LOT of changes, but it's icons all the way down.

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.WebSearch/FallbackExecuteSearchItem.cs

@@ -23,7 +23,7 @@ internal sealed partial class FallbackExecuteSearchItem : FallbackCommandItem
         Title = string.Empty;
         _executeItem.Name = string.Empty;
         Subtitle = string.Format(CultureInfo.CurrentCulture, PluginOpen, BrowserInfo.Name ?? BrowserInfo.MSEdgeName);
-        Icon = IconHelpers.FromRelativePath("Assets\\WebSearch.png");
+        Icon = Icons.WebSearch;
     }
 
     public override void UpdateQuery(string query)