Migrate to JSON configuration based extension registration

Retaining the UniversalLanguageSelector.php and Resources.php
to avoid breaking support for old MW versions. At some point,
we can validate how many old MW versions we need to support.

Bug: T87987
Change-Id: I006e968b70298b54013177aa792a4dfdf5b6aae2

UniversalLanguageSelector.php

@@ -18,6 +18,17 @@
  * @licence MIT License
  */
 
+if ( function_exists( 'wfLoadExtension' ) ) {
+	wfLoadExtension( 'UniversalLanguageSelector' );
+	// Keep i18n globals so mergeMessageFileList.php doesn't break
+	$GLOBALS['wgMessagesDirs']['UniversalLanguageSelector'] = __DIR__ . '/i18n';
+	/* wfWarn(
+		'Deprecated PHP entry point used for UniversalLanguageSelector extension. Please use wfLoadExtension instead, ' .
+		'see https://www.mediawiki.org/wiki/Extension_registration for more details.'
+	); */
+	return;
+}
+
 if ( !defined( 'MEDIAWIKI' ) ) {
 	echo "This file is an extension to the MediaWiki software and cannot be used standalone.\n";
 	die( -1 );