EN_NZHolidayStrategy: added Matariki and fixed 2011 Easter-Anzac clash

[b-h-mck]

• Added Matariki as a public holiday in New Zealand. Unfortunately it's a floating holiday with no readily-available algorithm, so I added it as a year map from 2022 (when it started) to 2035.
• Fixed (and added unit tests for) a bug where the NZ culture would crash for 2011 dates. This was happening because Easter Monday and Anzac Day were on the same day that year, and NZ legislation doesn't allow for any shifting in that scenario. I fixed it by combining them into a single holiday with both names.