fix(extensions)!: correct return type for subtract:date_iday operation (#1029)

BREAKING CHANGE: changes the return type of the `subtract:date_iday`
function from `date` to `precision_timestamp`

Author: nielspardon

extensions/functions_datetime.yaml

@@ -342,7 +342,7 @@ scalar_functions:
             value: date
           - name: y
             value: interval_day<P>
-        return: date
+        return: precision_timestamp<P>
   -
     name: "lte"
     description: less than or equal to

tests/cases/datetime/subtract_datetime.test

@@ -7,9 +7,9 @@ subtract(2016-12-01T13:30:15::pts<6>, P5Y::iyear) = 2011-12-01T13:30:15::pts<6>
 subtract(2016-12-01T13:30:15::pts<6>, PT5H::iday) = 2016-12-01T08:30:15::pts<6>
 
 # date: examples using the date type
-subtract(2020-12-31::date, P5D::iday) = 2020-12-26::date
+subtract(2020-12-31::date, P5D::iday<0>) = 2020-12-26T00:00:00::pts<0>
 subtract(2020-12-31::date, P5Y::iyear) = 2015-12-31::date
 subtract(2020-12-31::date, P5M::iyear) = 2020-07-31::date
 
 # null_input: examples with null args or return
-subtract(null::date?, P5D::iday) = null::date?
+subtract(null::date?, P5D::iday<0>) = null::pts?<0>