Add core/class_aliases.php

Problem/Motivation

In #3209618: [Symfony 6] Symfony\Component\HttpKernel\Event\KernelEvent::isMasterRequest() is deprecated, use isMainRequest() instead we added a class alias for Symfony\Component\HttpKernel\Event\KernelEvent in DrupalKernel. This works but it is not ideal because if you don't load the DrupalKernel class then the class alias is not in place. This isn't much of an issue normally but in unit tests it might mean the alias is not in place.

Proposed resolution

Add core/class_aliases.php as an autoload file in core/composer.json to ensure the class alias is always in place.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet