Use mb_strlen instead of grapheme_strlen

Due to inconsistent behave, mb_strlen will be used.

tests/Features/Expect/toHaveLength.php

@@ -5,7 +5,7 @@ use PHPUnit\Framework\ExpectationFailedException;
 it('passes', function ($value) {
     expect($value)->toHaveLength(9);
 })->with([
-    'Fortaleza', 'Sollefteå', 'Ιεράπετρα', 'PT-BR 🇵🇹🇧🇷😎',
+    'Fortaleza', 'Sollefteå', 'Ιεράπετρα',
     (object) [1, 2, 3, 4, 5, 6, 7, 8, 9],
     collect([1, 2, 3, 4, 5, 6, 7, 8, 9]),
 ]);