Handle conditional dates for liturgical events in national calendars

[JohnRDOrazio]

Certain events loaded from national liturgical calendars require some logic to create the event conditionally based on whether it falls on a Sunday or Solemnity. This logic is currently hardcoded into the main LitCalAPI class. An example is the National Day of Prayer for the Unborn in the USA:

https://github.com/Liturgical-Calendar/LiturgicalCalendarAPI/blob/436ebf79c93c65835c97e91232b0775bef60e8aa/includes/LitCalAPI.php#L2323-L2330

We should extend the national calendar schemas to allow for language constructs that will allow to define rules for how such situations should be handled. This will likely require a bit of thought and planning, based on various concrete examples.