Includes PHP 8+ compatibility check composer script.

This is an optional commit. It provides tools to help with compatibility checks for PHP 8+.

Ref recurly#637

composer.json

@@ -11,14 +11,25 @@
         "ext-curl": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "9.*"
+        "phpunit/phpunit": "9.*",
+        "squizlabs/php_codesniffer": "3.*",
+        "phpcsstandards/phpcsutils": "@alpha",
+        "phpcompatibility/php-compatibility": "dev-develop"
     },
     "autoload": {
         "classmap": ["lib"]
     },
+    "config": {
+      "allow-plugins": {
+        "dealerdirect/phpcodesniffer-composer-installer": true
+      }
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "2.4.x-dev"
         }
-    }
+    },
+  "scripts": {
+    "compatibility": "phpcs --extensions=php --severity=1 --standard=PHPCompatibility --runtime-set testVersion 8.1- lib Tests"
+  }
 }