This patch introduces is_interval() function for
require('datetime').interval.

Closes #10269

@TarantoolBot document
Title: Function require('datetime').interval.is_interval()

All modules representing field types, with the exception of `interval`
(i.e. `varbinary`, `uuid`, `decimal`, `datetime`), have an `is()` or
an `is_<type_name>()` function. Now such a function `is_interval()` is
provided for the `interval` module.