Allow workers & shared workers to be created within an extension service worker

[ian97531]

As Chrome extensions migrate to manifest v3, their background scripts become service workers. Since extensions cannot reliably spawn web workers from content scripts (due to potential CSP restrictions in the host page), it'd be very useful to be able to spawn web workers from the chrome extension's service worker to execute some WASM.

I originally filed this here but was encouraged to refile in this repo and repurpose the whatwg/html#8362 to focus on use cases that are relevant to the web platform.
For reference, enabling service workers to spawn web workers has also been discussed in 2 other (closed) issues whatwg/html#411 and w3c/ServiceWorker#678.

[dotproto]

Chromium is tracking this request in crbug.com/1219164.