Add wrapper function to check if a given role exists

[hlashbrooke]

Currently, the only way to check if a user role exists in the database is to use get_role() and check if the result is null or not. While that isn't too complicated to do, I think it would be useful to have a simple wrapper function that performs this check so that developers can easily check for a role before manipulating it (adding capabilities, etc.).

[hlashbrooke]

Adds role_exists() function that returns boolean true/false

[hlashbrooke]

Closing because this is unnecessary.

[F J Kaiser]

In case this ticket awakes again, here's a summary from ​the discussion in the Slack Channel.

1. Return strict and just the result as it evaluates to bool anyway.
2. Use \WP_Roles::is_role( $role )

Example:

/**
 * @param string $role
 * @return bool TRUE if the role exists
 */
function role_exists( $role )
{
    /** @var \WP_Roles $wp_roles */
    global $wp_roles;
    return $wp_roles->is_role( $role );
}