make the allowed values into constants

Probably should be an enum

I have submitted an MR that converts the allowed field label option values to constants inside FieldLabelOptionsTrait. No functional changes, only refactor to remove magic keys.

#4 seems like a good approach

Thanks for the feedback! I've updated the MR to use a PHP enum (`FieldLabelVisibility`) instead of constants, following the suggestion from @mstrelan and confirmed by @smustgrave. The trait now uses the enum's values for clarity and type-safety. Ready for re-review.

Code does what is claimed. Seems good to me. +1 RBTC