Improve dutch holiday naming

[janjongerden]

To match the commonly used Dutch naming, updated these days:

• 'king's day' -> 'Koningsdag'
• replaced 'STEPHENS' with 'SECOND_CHRISTMAS_DAY', as St.Stephens is not celebrated
• changed Easter, Pentecost and Christmas to use first/second day style ('eerste'/'tweede' in Dutch)

After this update, the list of holidays in the Netherlands, in Dutch will be:

Nieuwjaar
Dag van de koning --> Koningsdag
Bevrijdingsdag
Goede vrijdag
Pasen --> Eerste paasdag
Paasmaandag --> Tweede paasdag
Hemelvaartsdag
Pinksteren --> Eerste pinksterdag
Pinkstermaandag --> Tweede pinksterdag
Kerstmis --> Eerste kerstdag
St.Stephen's Dag --> Tweede kerstdag

For reference, two pages on Dutch holidays: Dutch government site and the wikipedia page.

[janjongerden]

Thank you for the quick review and merge @derTobsch 🎉 And for this handy library of course 👍