Port `Devise.add_module` from template repo

[tcannonfodder]

See: https://github.com/ruby-passkeys/devise-passkeys-template/blob/main/app/models/user.rb#L18

I'm pretty sure we can move this into the repo at the end of https://github.com/ruby-passkeys/devise-passkeys/blob/main/lib/devise/passkeys/model.rb

Alternatively, we could provide a helper method to create the add_module definition, but that feels like it's a bit too much configuration vs. following convention.

[tcannonfodder]

Closed because #17 addresses the root problem (lack of documentation)