polifill.php: add str_starts_with

inc/polyfill.php

@@ -0,0 +1,10 @@
+<?php
+
+// PHP 8.0
+
+if (!function_exists('str_starts_with')) {
+	function str_starts_with(string $haystack, string $needle): bool {
+		// https://wiki.php.net/rfc/add_str_starts_with_and_ends_with_functions#str_starts_with
+		return \strncmp($haystack, $needle, \strlen($needle)) === 0;
+	}
+}