[7.x] Optimize Str::startsWith() (#32243)

* optimize Str::startsWith by using strncmp

* add some tests with characters that are definitely utf8

Author: SjorsO

Str.php

@@ -604,7 +604,7 @@ public static function snake($value, $delimiter = '_')
     public static function startsWith($haystack, $needles)
     {
         foreach ((array) $needles as $needle) {
-            if ((string) $needle !== '' && substr($haystack, 0, strlen($needle)) === (string) $needle) {
+            if ((string) $needle !== '' && strncmp($haystack, $needle, strlen($needle)) === 0) {
                 return true;
             }
         }