Web worker code blocks

[Fil]

A js or ts code block could have an option that moves it to a web worker.

It might not be guaranteed to work (inputs and outputs must be serializable, among other constraints), but when it can work, it would be nice to make it as easy as adding a worker=true option.

Ref. https://observablehq.com/@fil/worker

[Fil]

Jason Miller's greenlet and workerize are useful helpers: https://observablehq.observablehq.cloud/pangea/party/greenlet