Composer: allow the PHPCS plugin

The `dealerdirect/phpcodesniffer-composer-installer` Composer plugin is used by the DMS Coding Standard to register external PHPCS standards with PHPCS.

As of Composer 2.2.0, Composer plugins need to be explicitly allowed to run. This adds the necessary configuration for that.

Refs:
* https://blog.packagist.com/composer-2-2/#more-secure-plugin-execution

Author: jrfnl

composer.json

@@ -24,5 +24,10 @@
         "psr-4": {
             "DMS\\PHPUnitExtensions\\ArraySubset\\Tests\\": "tests"
         }
+    },
+    "config": {
+        "allow-plugins": {
+            "dealerdirect/phpcodesniffer-composer-installer": true
+        }
     }
 }