Recommend the filter extension

Author: GrahamCampbell

composer.json

@@ -21,13 +21,17 @@
         "symfony/polyfill-ctype": "^1.9"
     },
     "require-dev": {
+        "ext-filter": "*",
         "phpunit/phpunit": "^4.8.35 || ^5.0 || ^6.0 || ^7.0"
     },
     "autoload": {
         "psr-4": {
             "Dotenv\\": "src/"
         }
     },
+    "suggest": {
+        "ext-filter": "Required to use the boolean validator."
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "3.6-dev"