(Replaces broken PR 1899...)

Prior to this PR, composer.json included library depotwarehouse/oauth2-twitch for OAuth with Twitch, but it did not have a wrapper.

However this library uses the Twitch Kraken API, which is no longer available, as of 2022 (source) - PHPLeague also need to deprecate the package, I'll let them know to remove it.

1. Removed the old provider and selected the replacement TwitchHelix wrapper from ThePHPLeague vertisan/oauth2-twitch-helix
2. Added this to the installer and stubs
3. Implemented the wrapper for TwitchHelix in the auth package
4. Updated documentation
5. Updated the authinstaller test
6. Created an Enum to provide the canonical name of the providers to keep naming conventions consistent throughout