Update composer configuration

- add MBString dependency
- update php-css-parser to 8.4

Author: bsweeney

composer.json

@@ -22,7 +22,8 @@
   },
   "require": {
     "php": "^7.4 || ^8.0",
-    "sabberworm/php-css-parser": "^8.3"
+    "ext-mbstring": "*",
+    "sabberworm/php-css-parser": "^8.4"
   },
   "require-dev": {
     "phpunit/phpunit": "^9.5"