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.Calc/CalculatorCommandProvider.cs

@@ -25,7 +25,7 @@ public partial class CalculatorCommandProvider : CommandProvider
     {
         Id = "Calculator";
         DisplayName = Resources.calculator_display_name;
-        Icon = CalculatorIcons.ProviderIcon;
+        Icon = Icons.CalculatorIcon;
         Settings = settings.Settings;
     }