idn_to_ascii function support

[soullivaneuh]

This is a feature request to add support for the idn_to_ascii function.

Ref: https://www.php.net/manual/en/function.idn-to-ascii.php

Return values: The domain name encoded in ASCII-compatible form, or false on failure

[shish]

So where nearly all functions are documented in <module name>/functions/<function name>.xml, idn_to_ascii and idn_to_utf8 are documented in intl/idn/<function name>.xml and so our code to find functions isn't finding them 😭