Add built-in time helper function "isPast"

Currently, the way to check if a time has passed is by doing:

LT(Time("2021-02-14T01:56:04.939970Z"), Now())

There should be a built-in function isPast that can accept an iso date string, it should return true if the given date is a past date, false if otherwise.

isPast("2021-02-14T01:56:04.939970Z") // true