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.WindowWalker/Commands/CloseWindowCommand.cs

@@ -20,7 +20,7 @@ internal sealed partial class CloseWindowCommand : InvokableCommand
 
     public CloseWindowCommand(Window window)
     {
-        Icon = new IconInfo("\uE894");
+        Icon = Icons.CloseWindow;
         Name = $"{Resources.windowwalker_Close}";
         _window = window;
     }