Add Async::Scheduler.enable_worker_pool and .worker_pool=

[ekmartin]

Summary

@ioquatix Do you think this makes sense? It was just a little clunky having to set the environment variable but we can also just wrap everything with our own reactor on our end if you don't think this is worth adding, that's completely fine too if you'd prefer keeping the surface smaller.

        Fiber.blocking do
          reactor = Async::Reactor.new(worker_pool: true)
          begin
            reactor.run(finished: false) { blk.call }.wait
          ensure
            Fiber.set_scheduler(nil)
          end
        end

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.