⭐ Enums for week days, months and units

[kylekatarnls]

If we drop PHP 8.0, using enums for week days, months and units is an option to be able write chaining in a more natural way, such as:

echo \Carbon\WeekDay::Monday->locale('it')->dayName; // lunedì
// equivalent to:
echo \Carbon\CarbonImmutable::parse('Monday')->locale('it')->dayName; // lunedì

echo \Carbon\Month::January->locale('it')->monthName; // gennaio
// equivalent to:
echo \Carbon\CarbonImmutable::parse('January 1st')->locale('it')->monthName; // gennaio

foreach (\Carbon\Unit::Week->stepBy('day') as $day) {
   $days[] = "$day";
}
// equivalent to:
foreach (\Carbon\CarbonInterval::week()->stepBy('day') as $day) {
   $days[] = "$day";
}

Those enums could also be supported in weekday(\Carbon\WeekDay|int $day) or month(\Carbon\Month|int $month)