Ensure the client can be used from a Web Worker

[RubenVerborgh]

Is your feature request related to a problem? Please describe. We should ensure the client can be used from a Web Worker, so request-heavy code (think queries) can run in a background thread.

A problem could be that LocalStorage is not available in workers, so we'd have to proxy credential access to the main thread.

Describe the solution you'd like

• add a test with a Web Worker
• possibly fix LocalStorage to make that test work

Describe alternatives you've considered

• main thread, but that can make things sluggish

Additional context

[nicolasmondada]

#947 should be considered when adding support for Web Workers.