add support for multiple simultaneous task queues

The initial prototypes of the task queue system demonstrated the concept of running work in WebWorkers and equivalent Node workers in Google Cloud. That demo only ran when the task queue was open as the current tool: now what we need to do is to support multiple queues and run them off-screen.

Alex and I have a prototype of a tool which hosts a task queue in a tray-like runner but doesn't support multiple queues.

Designing the task-runner system to support working different queues from different documents with differing priorities -- and designing them to be resilient against accidental fork bombs and starvation... it's an interesting puzzle!