Improve holiday definitions for The Netherlands

[ehoogeveen-medweb]

• Changed type of Goede Vrijdag (Good Friday) from public to school
• Changed type of Bevrijdingsdag (Liberation Day) from observance to school
• Set name of Ascension Day to Hemelvaartsdag as an override
  • Belgium has O.L.H. Hemelvaart and O.L.V. Hemelvaart, but in the Netherlands we only have the former and it's called Hemelvaartsdag in official sources.
  • The official Belgian names are unclear as Hemelvaartsdag and Maria-Hemelvaart are also used in official sources (compare https://www.belgium.be/nl/over_belgie/land/belgie_in_een_notendop/feestdagen and https://www.belgium.be/nl/werk/verlof_en_loopbaanonderbrekingen/feestdagen). I decided not to mess with the overall Dutch translation for Ascension Day.
• Add historical definitions for Koninginnedag (Queen's Day) and Princessedag (Princess Day)
  • Queen's Day was celebrated prior to 2014 so it might have real world relevance. I added the older variants for completeness.
• Add English translations for Bevrijdingsdag (Liberation Day) and Prinsjesdag (Prince's Day)

Technically several of these holidays can be seen as "optional" as there's no legal requirement, but most people are free and they're school holidays as well, so I left them as public. Let me know if it would be better to change them.

Fixes #434.

[commenthol]

@ehoogeveen-medweb, Great, many thanks.