[php-src] PR #23661: ext/standard: Optimize str_pad() using doubling copies

Pull Request: https://github.com/php/php-src/pull/23661 Author: LamentXU123 I was reading standard code recently. The implementation of str_pad here is quite old that we make a loop to pad the strings if the remaining padding length is smaller than padded-string's length. In modern implementations like OpenJDK for example, we use a doubling algo for this: https://github.com/openjdk/jdk/blob/jdk-21%2B35/src/java.base/share/classes/java/lang/String.java#L4682 This is way more faster than the original one. Let's say we want to pad "ab" for 1 MiB. With the original one we need ~52,000 times of memory copying but with this implementation we only need 20 times.