Feature request: Add "disabled" mode to run without using threads

Use case: Debugging threads is possible, but harder than debugging the main process. If I have a problem in one of my threads, or the pool entry code itself, it would be amazing if I could provide threads.js an option or env var to disable threading. This way I don't have to refactor my code, I can use the same pool queuing code/event subscribing/thread expose code, but run it all single-threaded so I can set a debugger anywhere in the pipeline.

I'm aware a feature like this would be a fair amount of work by reworking `expose` to act like `module.exports` for the main thread instead, but I wanted to post this in case anybody likes this idea.