Don't load mediawiki.ui.button on page load

The stylesheet is needed on legacy Vector but not modern
Vector 2022 meaning Vector 2022 loads two sets of styles
for buttons.

This is a short term fix until UniversalLanguageSelector can
use Codex.

Bug: T340255
Change-Id: Ib4e228908d2813898654886bc1ee76e90dc52f22

resources/js/ext.uls.compactlinks.js

@@ -190,7 +190,9 @@
 			this.compactList[ language ].parentNode.style.display = '';
 		}
 
-		this.addTrigger();
+		mw.loader.using( 'mediawiki.ui.button' ).then( function () {
+			this.addTrigger();
+		} );
 
 		mw.hook( 'mw.uls.compactlinks.initialized' ).fire( true );
 	};