Add Teachable OAuth provider

[ralphsmith80]

Goals

1. Add Teachable as an available OAuth provider
2. Allow the use of Teachable OAuth with AuthJS with minimal configuration

Non-Goals

No response

Background

Teachable is a common platform used by many content creators. This functionality would enable a more seamless integration when using AuthJS to create a custom site around the Teachable content.

Proposal

It should follow the current patterns implemented by other providers.
https://docs.teachable.com/docs/oauth-quickstart-guide