Add enum_cases function

[HeahDude]

May fix #3681.

Allows:

{% for enum in enum_cases('App\\MyBackedEnum') %}
    {{ enum.value }}
{% endfor %}

---

Use case:
We need to display supported locales for a language switcher in the menu.
The block is included in a header template which is included in the base. It is cumbersome to pass Locale::cases() in the context from all controllers rendering a template that inherits from the base, or to cascade the variable from template to template.

Simplified implementation:

enum Locale: string
{
    case EN = 'en';
    case FR = 'fr';

    public function flag(): string
    {
        return match ($this) {
            self::EN => '🇬🇧',
            self::FR => '🇫🇷',
        };
    }
}

{% for locale in enum_cases('App\\Locale') %}
    <option value="{{ locale.value }}"{% if locale.value == app.locale %} selected{% endif %}>
        {{ locale.flag }}
    </option>
{% endfor %}

[HeahDude]

I've updated the description to show a basic use case.

[GromNaN]

The restriction on BackedEnum is not necessary. As we can see in you Locale::flag example, the enum case can be used as argument to call a function, or directly to call a method on it.

[GromNaN]

Even if you put an if around it, enum is a syntax error for PHP < 8.1. The @requires PHP 8.1 doesn't help either since the file can't be compiled on PHP 8.0 and bellow. That should be OK if you move this enum declarations in other files.