[TASK] Ensure this package gets installed as dev dependency

The keyword "dev" [1] causes Composer to warn if this package
gets installed as a production dependency instead of as a
development dependency.

This supports developers in installing this package correctly.

[1] https://getcomposer.org/doc/04-schema.md#keywords

Author: oliverklee

composer.json

@@ -7,6 +7,7 @@
 		"typo3",
 		"cms",
 		"code style",
+		"dev",
 		"editorconfig",
 		"php-cs-fixer"
 	],