DEV: Move providers into their own module

[nattsw]

Note: The diff numbers are wildly incorrect since this is just file moves.

Historically, we've had this structure

- discourse-translator
  - base.rb
  - google.rb
  - discourse-ai.rb
  - ...other providers

Any translation needed is invoked on the base class. And the base class' .translate method needs to account for both topics and posts.

As we're moving to translate more types, we need to move away from this. This PR is encapsulates the providers listed above into their own module so we can have a separate translation class altogether instead of calling Base.

[Drenmi]

Thanks for separating the refactoring and the feature work. 👌👨‍🍳