Add "dev" Composer keyword

As per https://getcomposer.org/doc/04-schema.md#keywords by including
"dev" as a keyword in the `composer.json` file, Composer 2.4.0-RC1 and
later will prompt users if the package is installed with `composer
require` instead of `composer require --dev`. See
composer/composer#10960 for more info.

Author: GaryJones

composer.json

@@ -6,6 +6,7 @@
         "doctrine",
         "coding",
         "standard",
+        "dev",
         "cs",
         "code",
         "style",