Add schedule:work as crontab alternative

[francoism90]

I think this would be a good addition. The scheduler can be run using a single command and without a cronjob.
I have also added priority, as the php-fpm should be, in most cases, the one started first and stopped last.

Let me know your thoughts, if this should be in the docs instead, I'm happy to do this as well. :)

Thanks.

[taylorotwell]

You can add this to your own Sail files if you wish. Or just run schedule:work in another console tab.