Add special dev to composer keywords

[jnoordsij]

The special dev keyword will warn users who run composer require barryvdh/laravel-debugbar with the following prompt:

> composer require barryvdh/laravel-debugbar
The package you required is recommended to be placed in require-dev (because it is tagged as "dev") but you did not use --dev.
Do you want to re-run the command with --dev? [yes]?

See also https://getcomposer.org/doc/04-schema.md#keywords.