Make available field retrieval be generated through an object

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

public/csv.php

@@ -31,7 +31,8 @@
 }
 else $selectionActive = false;
 
-require __DIR__ . "/../values/availableFields.php";
+$fieldsGetter = new CsvxmlAvailableFields($lang);
+$availableFields = $fieldsGetter->getFields();
 
 $line1 = $line2 = $line3 = $line4 = [];