Method utf8_find_nth

include/stringzilla/stringzilla.hpp:4191–4197

* @brief Find the byte offset of the nth UTF-8 character. * @param n The character index (0-based). * @return The byte offset, or npos if n >= character count. */

     *  @return The byte offset, or npos if n >= character count.
     */
    size_type utf8_find_nth(size_type n) const noexcept {
        sz_ptr_t start;
        sz_size_t length;
        sz_string_range(&string_, &start, &length);
        auto ptr = sz_utf8_find_nth(start, length, n);
        return ptr ? ptr - start : npos;
    }

    /**
     *  @brief Iterate over UTF-8 characters (codepoints) in the string.