feat: introduce Constructor attribute

[romm]

This attribute can be assigned to any method inside an object, to automatically mark the method as a constructor for the class. This is a more convenient way of registering constructors than using the MapperBuilder::registerConstructor method, although it does not replace it.

The method targeted by a Constructor attribute must be public, static and return an instance of the class it is part of.

final readonly class Email
{
    // When another constructor is registered for the class, the native
    // constructor is disabled. To enable it again, it is mandatory to
    // explicitly register it again.
    #[\CuyZ\Valinor\Mapper\Object\Constructor]
    public function __construct(public string $value) {}

    #[\CuyZ\Valinor\Mapper\Object\Constructor]
    public static function createFrom(
        string $userName, string $domainName
    ): self {
        return new self($userName . '@' . $domainName);
    }
}

(new \CuyZ\Valinor\MapperBuilder())
    ->mapper()
    ->map(Email::class, [
        'userName' => 'john.doe',
        'domainName' => 'example.com',
    ]); // [email protected]

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Fixes #99