Add mbstring dependency to composer.json

Author: Schrank

composer.json

@@ -24,7 +24,8 @@
     "homepage": "https://www.sabberworm.com/blog/2010/6/10/php-css-parser",
     "require": {
         "php": "^7.2.0 || ~8.0.0 || ~8.1.0 || ~8.2.0 || ~8.3.0 || ~8.4.0",
-        "ext-iconv": "*"
+        "ext-iconv": "*",
+        "ext-mbstring": "*"
     },
     "require-dev": {
         "php-parallel-lint/php-parallel-lint": "1.4.0",