Implement Postal Code Generator using Positional Generator for locale `nl`

In lib/locales/nl.yml, there is a very large definition for a generator with over 2000 characters to be used by the regexify method.

My understanding, based on this discussion is that any combination of 2 uppercase characters are valid, except for SA, SS, and SD.

Instead of implementing a way to exclude the invalid combinations, the locale specifies all the 673 repeated permutations of possible 2-letter words that are valid. This seems very excessive to me.

AA, AB, AC, ... ZZ.

I think we could simply implement this with a generator for the postal code excluding using the positional generator instead.