[8.x] Add class argument

[jasonmccreary]

This is a retry of #35536 after getting some love on Twitter. It avoids the breaking change by preserving the current --class option and adding a class argument which will be used if set. Otherwise, existing behavior ensues.

For example, all of the following may be used to run the DatabaseSeeder.

php artisan db:seed --class=DatabaseSeeder
php artisan db:seed DatabaseSeeder
php artisan db:seed

For a specific seeder, you may run:

php artisan db:seed --class=UsersTableSeeder
php artisan db:seed UsersTableSeeder