turbo-tasks: add scope_self_feeding, a self-feeding parallel scope

A self-feeding parallel scope: jobs run on a shared work-queue and any job may
enqueue more, drained by the runtime workers plus the calling thread. Deadlock-safe
on thread-limited/contended runtimes (helpers are a pure optimization, not required
for progress). Used to parallelize work pools that discover more work as they run.

Author: lukesandberg