turbo-tasks: add scope_unbounded, a work-stealing scope with a growing work queue

A parallel scope where the `run` closure receives a `Spawner` and may enqueue more
items while the scope drains — unlike `scope_and_block`'s fixed job set. Completes once
every item (initial + transitively spawned) is processed; termination is driven by an
in-flight counter reaching zero.

Deadlock-safe on thread-limited/contended runtimes: the calling thread drains the whole
(growing) queue itself if no helper task is scheduled, so liveness never depends on a
helper. Needed to parallelize work pools that discover more work as they run (the GC
collection pass, later in the stack).

16 unit tests cover the drain loop, the empty/tree/panic cases, and the
current-thread-runtime (zero-helper) liveness property.

Author: lukesandberg