Experiment with activating `onStartupFinished` extensions with a timeout

[joyceerhl]

Right now we do it right on startup, and there might not be an immediate user gain if an extension moves from * to onStartupFinished.

[Tyriar]

Should this use an idle callback instead of setTimeout?

[joyceerhl]

It seems we can't use idle callbacks:

• idle callbacks are only available in a window context, so using runWhenIdle falls back to setTimeout0 in this context
• the api docs say that we should avoid using idle callbacks for tasks which take an unknown time to run, and extension activation times do significantly vary