add support for php8.x

[horaciod]

After doing some analysis of the errors found when running the tests in php 8.1 I made some changes to respect the strict types that are now required as return in SplDoublyLinkedList

avoiding +Fatal error: Uncaught TypeError: File_MARC_List::key(): Return value must be of type int, string returned in ... /MARC/List.php:101\n

I'm using integers instead of strings for keys in linked lists. It is only necessary to convert the keys in the case of using them directly as in marc_list_001.phpt. Also fixed some issues with parameters sent to the preg_match method.