Add new holibobs for Day of Mourning

[pcraig3]

We are getting a new one-time (at least) federal holiday this year, the Day of Mourning to commemorate the passing of the Queen. As of this point, it's just for federal employees, but provinces may also come on board.

This is a one-time holiday, it will not be observed in past years or next year.

Here's the article: https://www.cbc.ca/news/politics/queen-funeral-national-holiday-or-not-1.6580582

Notes

• The previous commit I had where I added Truth and Reconciliation was great here: https://github.com/pcraig3/hols/pull/94/files
• Added a new nullable field (lastOccurence)
  • Essentially, this holiday is first observed this year and last observed this year.
  • No other holidays are using this field yet, but who can say what will happen in the future?

Screenshot

New screenshot has the new holiday.

before	after