Add holidays for Luxembourg

[UrbinCedric]

Another small contribution, just like the country itself!

Useful sources for validation:

• Offical Gov: https://luxembourg.public.lu/en/living/quality-of-life/jours-feries-legaux.html
• And for the luxembourgish holiday names: https://feiertag.info/feiertage/luxemburg/

Thanks for all your efforts Spatie, Much love from Luxembourg! ❤️😄

[Nielsvanpach]

Thanks!