add Optional attribute that allows specifying default value

[JanTvrdik]

Resolves #59.

Add alternative approach to dealing with undefined values, that does not require wrapping values with generic Optional. This makes it impossible to distinguish between undefined and default value (usually null), but is overall simpler to deal with.

readonly class Semaphore
{
    public function __construct(
        #[Optional(default: SemaphoreColorEnum::Red)]
        public SemaphoreColorEnum $color,

        #[Optional]
        public ?string $manufacturer,
    ) {
    }
}

[janedbal]

Readme change missing