Clean up yield inheritance

[shaseley]

1. Key the scheduling state based on the {{Scheduler}} to prevent leaking it across (potentially cross-origin) windows. This changes the event loop's continuation state to be a small wrapper around a map. The continuation state is propagated in the same way, but the scheduler state is unique to the scheduler and not shared. In practice there will only be one entry in this map (a task or microtask can only have originated from one task), but the mechanism is generic enough to support other use cases, implementations can optimize this, and the key/value mapping hopefully makes the isolation clear.

   Alternatively, we could propagate only the state for the current scheduler, but we don't always know the current scheduler, e.g. in "queue a microtask", and this model is different enough from AsyncContext and Chrome's "Task Attribution" that we'd need a separate mechanism, which is a performance concern. The main behavioral difference is how propagating is handled in the case of A --> (B microtask) --> A. With this approach, the context is preserved in the second call to A, which matches the synchronous behavior of A --> calls B --> calls A.

2. Propagate the current scheduling state in "queue a microtask", unless coming from JavaScript, in which case the propagation is handled by the abstract job closure. Previously, the state would be inherited only if it wasn't reset by another microtask or after the postTask callback ran. This fixes the inconsistency, making directly scheduled microtasks match microtasks originating from JavaScript.

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Preview | Diff

[shaseley]

Hi @smaug---- would you mind taking a look? I'm going to start migrating stuff to HTML, which will make the monkey patching easier to reason about, but I wanted to have a starting point. The indirection is maybe a little overkill, but I think having the map makes the isolation clear, and it can probably be expanded later to support AsyncContext since the microtask propagation should be the same, but TBD.