Make available field retrieval be generated through an object

phpcs-errors:221 phpunit-status:successful phpstan-errors:677

public/index.php

@@ -57,7 +57,8 @@
     </div>
 ';
 
-require __DIR__ . "/../values/availableFields.php";
+$fieldsGetter = new CsvxmlAvailableFields($lang);
+$availableFields = $fieldsGetter->getFields();
 
 $tooltips = [];
 foreach ($availableFields as $headline => $fields) {