Support the building of an executable which has optional multithreading if `SharedArrayBuffer` is available

For [`magickwand.js`](https://magickwand.js.org/) I would like to be able to ship an universal binary that uses multithreading and has `xxxAsync` methods available when `SharedArrayBuffer` is available but which can be loaded without `SharedArrayBuffer` - async methods will throw in this case.

This will leave the user the option to implement multithreading himself - by creating web workers and passing messages - eventually using a framework such as [`comlink`](https://www.npmjs.com/package/comlink).

As this will require significant changes, I would like to discuss it before starting the implementation.