Move "the worker's ports" into the WorkerGlobalScope section

[annevk]

It's clearer if we have all internal slots in one place. A related thing that might be nice to do is to more clearly stipulate how this set is populated. (It also doesn't seem to define that ports can go away other than on shutdown. That's probably wrong since you can disentangle ports and if you don't hold references to them and they don't have an event listener registered the user agent could collect them as well.

Follow-up from #2520.