[php-src] PR #23533: ext/intl: report argument #3 ($variant) for an invalid IDNA variant

Pull Request: https://github.com/php/php-src/pull/23533 Author: lacatoire php_intl_idn_handoff() parses $domain, $flags, $variant, so the variant check must report argument 3. It hardcodes 2, so idn_to_ascii() and idn_to_utf8() blame $flags for a value that was passed as $variant. Before: ``` ValueError: idn_to_ascii(): Argument #2 ($flags) must be INTL_IDNA_VARIANT_UTS46 ``` After: ``` ValueError: idn_to_ascii(): Argument #3 ($variant) must be INTL_IDNA_VARIANT_UTS46 ``` Only the error message changes. The existing test is updated and a named-argument case is added, where the wrong number was the most misleading.