ext/pdo_firebird: Do not implement new constants in PDO core (#14995)

Implement PDO driver-specific subclasses

RFC: https://wiki.php.net/rfc/pdo_driver_specific_subclasses
Closes GH-12804

Co-Authored-By: Danack <Danack@basereality.com>

Feature: ext/pdo_firebird: Add transaction isolation level and mode settings (#12815)

* Added transaction isolation level and access mode

* Raise a ValueError if an invalid value i

Feature: ext/pdo_firebird: Add transaction isolation level and mode settings (#12815)

* Added transaction isolation level and access mode

* Raise a ValueError if an invalid value is passed to PDO::FB_TRANSACTION_ISOLATION_LEVEL.

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