[php-src] PR #25179: [RFC] Add grapheme_str_pad() function

Pull Request: https://github.com/php/php-src/pull/25179 Author: ChrisAbner RFC: https://wiki.php.net/rfc/grapheme_str_pad Adds grapheme_str_pad() to ext/intl, the grapheme cluster counterpart of str_pad() and mb_str_pad(). ```php function grapheme_str_pad(string $string, int $length, string $pad_string = " ", int $pad_type = STR_PAD_RIGHT): string|false {} ``` - The target length and the input length are counted in grapheme clusters, like grapheme_strlen(). - The padding string is repeated and its last copy is only cut on a grapheme cluster boundary. - Errors follow mb_str_pad() (ValueError for an empty padding string or an invalid $pad_type, no error when nothing needs padding). Invalid UTF-8 returns false and sets the intl error, like grapheme_strlen(). - ASCII input takes a fast path and gives the same result as str_pad(). Tests: ext/intl/tests/grapheme_str_pad_basic.phpt (NFC/NFD, combining dakuten, Devanagari, emoji with modifier and ZWJ sequences, cuts of the padding string, "\r\n") and grapheme_str_pad_errors.phpt.