Add method `Manager::hasRole(string $name): bool`

[vjik]

[beastbytes]

As Roles and Permissions are both Items, and Item names must be unique,
I think hasItem() would be a better method name.

[vjik]

As Roles and Permissions are both Items, and Item names must be unique, I think hasItem() would be a better method name.

Using hasItem() method, we will not be able to find out that the existing item is exactly a role.