Implement worker scaffolding to enable parallelization

[TalDerei]

this work stream is well summarized by this comment. After completing this, we'll be able to parallelize the trial decryption since the polyfill now allows us to launch workers for individual sub-tasks, for instance transaction building.

while the polyfill uses offscreen, the rest of the services package wasn't launched that way and is still running in the background script in the service worker. In the present situation, everything — including all top-level tasks not launched with new Worker— runs on the main extension thread. However, after this phase, services no longer use chrome.runtime and could be moved into the offscreen entry in a separate phase.

[TalDerei]

closing in favor of #1787