move AuthTokenFile to normal instantiated class
AuthTokenPathname can determine existence of tokens

The tokens are matched both with and without github_url suffix
use AuthTokenPathname to determine XDG or legacy
AuthTokenPathname can be converted to a Pathname

The pathname includes the github_url suffix, as appropriate
define to_s and define to_pathname in terms of to_s
AuthTokenFile now uses AuthTokenPathname for generating pathnames
AuthTokenFile need not know about github_url_suffix
attributes
spec cleanup