Enforce UpperCamelCase case values in PHP enum definitions

Problem/Motivation

At #3339746: Coding style for PHP Enumerations → we decided to use UpperCamelCase for the case labels in PHP enum definitions.

Although SlevomatCodingStandard has a few sniffs related to PHP enums, some of which we're already going to enable and use, I can't find one that will enforce a certain case for the case labels (apologies for the overloaded "case" terminology). 🙃

For now, opening this here in coder. We can move it if there's a better way to enforce this.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes