Enum case casing sniff

[dereuromark]

I saw the following sniffs around enums

    <rule ref="SlevomatCodingStandard.Classes.BackedEnumTypeSpacing"/>
    <rule ref="SlevomatCodingStandard.Classes.EnumCaseSpacing"/>

But I think it would also be a good idea to have the recommended casing for enum cases to be added Refs https://www.php.net/manual/en/language.enumerations.backed.php

See also

• https://github.com/php-fig/per-coding-style/blob/master/spec.md#9-enumerations
• https://symfony.com/doc/current/contributing/code/standards.html

enum Suit: string
{
    case Hearts = 'H';
    case SOME_SPADES = 'S';
}

should be fixed to

enum Suit: string
{
    case Hearts = 'H';
    case SomeSpades = 'S';
}

with a warning as this would probably be not safe to change.

We could also just issue an error instead of fixable error for existing underscored ones.