Multiple Platforms

What about having multiple platforms in one repo? Most developers don't want to make a repo for each platform. They should just be able to put all the classes in one repo, maybe an `ios` folder and `android` folder, and then the server can take the contents of that folder and put that in the repo.

AND! If we use folders like that, then we don't even need to specify platforms in the `jr.json`. The server can just detect if there's an `ios` folder or an `android` folder and infer what platforms the extension supports from there.

Start by tracing how the server currently reads jr.json and assembles repository contents. Compare that flow with the proposed ios and android folders, then determine how platform detection and multi-platform packaging should work. Done means the behavior and configuration changes are specified and supported consistently.

Written by the indexing model from the issue text.