Aggregate the JS and JVM projects to simplify builds.

Currently, commands such as `compile` and `publish` do not propagate to the JS and JVM projects, requiring separate `fooJS/compile` and 'fooJVM/compile` etc commands. By making the top-level project aggregate the JS and JVM projects, such commands now automatically propagate to the subproject.

It also avoids publishing an empty jar for the root project (foo-root-project) when `publish` is invoked at the root project. Instead, now it correctly delivers `foo` and `foo_sjs` jars.