fix: Allow Workers to be instantiated directly via callback

[Jarbuckle]

fix: Allow Workers to be instantiated directly via callback

What

When testing out Worker Pools in production builds, the Workers were not being instantiated as expected because the build system (Vite, in this case) was treating the imported worker modules as the video/mp2t MIME type. After some digging, this was apparently due to the Vite bundler expecting a worker instantiation to be done in a very specific (and not particularly flexible) way.

However, Vite did offer an alternative that gave a clue: a worker module could be imported with ?worker at the end of the filename to import it instead as a constructor that could be called, and the docs hinted at the possibility that this could then be used in contexts where the URL of the worker wasn't known.

Taking this idea one step further, this PR makes it so that the Worker Pool can accept either a URL (possibly only a temporary thing, since it may never work...) OR a Worker instantiation callback. This was tested using Vite's worker constructor within the callback, and it worked within production builds as well as locally.

How

• Replace the URL parameter to Worker Pool instantiation with a WorkerInit object, which can be either a URL or a WorkerInstantiationCallback. A WorkerInstantiationCallback is an parameter-less function that returns a Worker.

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