Auto merge of #21943 - codehag:devtools-add-list-workers-method, r=jdm

DevTools - add ListWorkers method to BrowsingContextTargetActor

This is one of three pull requests that allows the DevTools Debugger to render.

The two related prs are #21942 and #21944

This pr introduces a `ListWorkers` method, which the debugger relies on for startup. At the moment, we are returning an empty array, but later on we can return an array populated by all workers associated with a target.

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