Add Scotland and Northern Ireland public holidays

[davidxmoody]

Hi, I've added public holiday definitions for Scotland and Northern Ireland following the existing examples.

When creating these, I ran into a couple of things I wasn't sure how to deal with. These may be larger issues which could be saved for a different pull request.

1. In 2022 only, there's a UK-wide public holiday for the Queen's Diamond Jubilee. I couldn't spot another example of a one year only public holiday so didn't include it for now.
2. I wasn't sure the best way to handle substitute bank holidays. In the UK whenever a bank holiday would fall on a weekend, it gets moved to the next available weekday. There were some examples of this already happening (with the nextMo-Fr17March syntax). However I would have expected it to occur for every holiday (including Christmas, etc.). In this pull request I decided to follow the existing examples and leave the Christmas/New Year ones as fixed dates but add the other variable ones correctly shifting to the next available Monday.

[ypid]

Thanks. The long term goal is #300, so I would ignore 1. for now. I also ignored some 1 year only PH in Germany. About 2, I have no local knowledge of the UK but I have seen some discussion about bank holidays in regards to the opening_hours syntax. Some clear definition is needed how we (OSM) want to handle this. Can you check for such discussions and if consensus was reached on it? A proposal process could be good also. For now it is good that you follow the existing example.