Revert "Speed up splitting strings into chars"

That function is only available in PHP 7.4

This reverts commit 56fb7a7.

Author: colinodell

src/Parser/Cursor.php

@@ -247,7 +247,7 @@ public function advanceBy(int $characters, bool $advanceByColumns = false): void
             $asArray = [$nextFewChars];
         } elseif ($this->isMultibyte) {
             /** @var string[] $asArray */
-            $asArray = \mb_str_split($nextFewChars);
+            $asArray = \preg_split('//u', $nextFewChars, -1, \PREG_SPLIT_NO_EMPTY);
         } else {
             $asArray = \str_split($nextFewChars);
         }

src/Util/UrlEncoder.php

@@ -29,7 +29,7 @@ public static function unescapeAndEncode(string $uri): string
         $result = '';
 
         /** @var string[] $chars */
-        $chars = \mb_str_split($uri);
+        $chars = \preg_split('//u', $uri, -1, \PREG_SPLIT_NO_EMPTY);
         $l = \count($chars);
         for ($i = 0; $i < $l; $i++) {
             $code = $chars[$i];