Redefine non-standard DATE_ADD and DATE_DIFF to interval arithmetic

[RCHowell]

Description

Once #1657 is completed, then we can redefine DATE_ADD and DATE_DIFF to be special syntax for the standard SQL datetime arithmetic operators.

This is outside the scope of updating Datum to use java.time, so existing DATE_ADD and DATE_DIFF only matches 0.14 behavior which is not ideal and wrong in some places.