Install slevomat/coding-standard as dev dependency

composer.json

@@ -27,7 +27,8 @@
     "phpunit/phpunit": "^9",
     "mockery/mockery": "^1.6",
     "phpstan/phpstan": "^1",
-    "squizlabs/php_codesniffer": "^3.8"
+    "squizlabs/php_codesniffer": "^3.8",
+    "slevomat/coding-standard": "~8.0"
   },
   "suggest": {
     "ext-exif": "Recommended to be able to read EXIF data properly."
@@ -41,5 +42,10 @@
     "psr-4": {
       "Intervention\\Image\\Tests\\": "tests"
     }
+  },
+  "config": {
+    "allow-plugins": {
+      "dealerdirect/phpcodesniffer-composer-installer": true
+    }
   }
 }