Switch WebWorker handling over to comlink

This library is approximately the same age as https://github.com/GoogleChromeLabs/comlink#readme and contains a homegrown version of much of the functionality that comlink provides. Switching to comlink for WebWorker interactions should allow a lot of code to be eliminated from Loam and provide a friendlier internal interface, to boot.

Start by reviewing Loam's existing WebWorker interaction code and comparing its homegrown functionality with comlink. Identify the code that can be removed and the internal interfaces that must be migrated; done means WebWorker interactions use comlink without regressions.

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