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.WindowsTerminal/TerminalTopLevelCommandItem.cs

@@ -14,7 +14,7 @@ public partial class TerminalTopLevelCommandItem : CommandItem
     public TerminalTopLevelCommandItem(SettingsManager settingsManager)
         : base(new ProfilesListPage(settingsManager))
     {
-        Icon = WindowsTerminalCommandsProvider.TerminalIcon;
+        Icon = Icons.TerminalIcon;
         Title = Resources.list_item_title;
     }
 }