[pending-task] Cancellation mechanism about community-protocols

[justinfagnani]

I see the desire for this, but I think it's worth considering the existing ways to accomplish cancellation and whether adding more is good or not, and whether cancellation should be tied to pending-tasks - are there cases where you want to cancel that wouldn't use pending-task?

I can think of two main ways to do this now:

1. Use a concrete scheduler API. If there is some overarching controller of tasks, there's a good argument that tasks shouldn't just be implicitly created under it with minimal control (cancellation), but that tasks should be created in coordination with the scheduler so it can fully control them (pause/resume, logging, progress, debounce, etc.)
2. AbortSignal. This is already a way to cancel and can be thread into other APIs like fetch(). The interoperability here is compelling to me.

Both of these could be provided via either a context or properties.

One thing that's interesting about AbortSignal is the potential to compose them: An async operation could consume one at a standard context key (ie, 'abort-signal' - which could be a small community protocol itself), and any ancestor that wanted to cancel could provide it, and could also consume a signal from above and combine it with its own additional signals to create nesting cancellation.