add require ext-mbstring

Author: lcp0578

composer.json

@@ -31,7 +31,8 @@
         "stefanak-michal/bolt": "^5.1",
         "symfony/polyfill-php80": "^1.2",
         "psr/simple-cache": ">=2.0",
-        "ext-json": "*"
+        "ext-json": "*",
+        "ext-mbstring": "*"
     },
     "provide": {
         "psr/simple-cache-implementation": "2.0|3.0"