make the allowed values into constants

Problem/Motivation

Follow up from 📌 Move \Drupal\field_ui\Form\EntityViewDisplayEditForm::getFieldLabelOptions() to a trait Fixed post from there

\Drupal\field_ui\Form\EntityViewDisplayEditForm::getFieldLabelOptions() contains code that is essential to configuring a field formatter, with 4 magic keys, yet it is in a protected method on a single form implementation.

Steps to reproduce

TBD

Proposed resolution

make the allowed values into constants

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet