Fix PHPStan missingType.generics for FieldItemListInterface not specifying its type

phpstan-drupal has a stub that defines FieldItemListInterface as a generic, so we need to pass the type in the PHPDocs here.

See https://github.com/mglaman/phpstan-drupal/blob/main/stubs/Drupal/Core/Fi...

TBH I think it’s nonsense to have core lacking appropriate PHPDoc, for drupal-phpstan to write a stub gapfill, and then that gapfill to force back core implementations’ PHPDocs to be adjusted…

I would suggest to move the definitions in the stub from drupal-php to core as part of this issue.

phpstan-drupal uses a stub provider, so we can fix here and back port anything to phpstan-drupal and use a version check on Drupal to decide if the phpstan-drupal stub should be loaded or not

Done #7 for the Drupal core part (I think).

Left a comment on the MR but leaving in review for someone more familiar

Thanks for the response @mglaman. Moving to NW for rebase.

If you are another contributor eager to jump in, please allow the original poster @mglaman or @mondrake at least 48 hours to respond to feedback first, so they have the opportunity to finish what they started!

Rebased, only merge conflicts resolved.

Rebase seems good.