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/Commands/LaunchProfileAsAdminCommand.cs

@@ -29,7 +29,7 @@ internal sealed partial class LaunchProfileAsAdminCommand : InvokableCommand
         this._openQuake = openQuake;
 
         this.Name = Resources.launch_profile_as_admin;
-        this.Icon = new IconInfo("\xE7EF"); // Admin icon
+        this.Icon = Icons.AdminIcon;
     }
 
     private void LaunchElevated(string id, string profile)