Add "dev" keyword

This will make Composer recommend this package for "require-dev".

https://php.watch/articles/composer-prompt-require-dev-dev-packages

Author: ravage84

composer.json

@@ -2,7 +2,7 @@
     "name": "cakephp/debug_kit",
     "description": "CakePHP Debug Kit",
     "type": "cakephp-plugin",
-    "keywords": ["cakephp", "debug", "kit"],
+    "keywords": ["cakephp", "debug", "kit", "dev"],
     "homepage": "https://github.com/cakephp/debug_kit",
     "license": "MIT",
     "authors": [