Support service closure shorthand syntax in YamlFileLoader

Postponing until this is available in a supported symfony version.

Hardly needs to be postponed, Drupal has a copy of YamlFileLoader, the patch could be applied any time.

Documentation is underway too.

Yeah we can still do this, no need to wait years.

https://symfony.com/releases/7.3 comes out in May

https://www.drupal.org/about/core/policies/core-release-cycles/schedule → 11.2 comes out in Jun

if this gets in now, a released Symfony still will predated a released Drupal

(not that I have any idea why would that be a prerequisite)