Reject pending promises when a generator is closed

[tomlarkworthy]

This treats an issue raised here observablehq/runtime#376

V6 runtime will wait for prior promises to resolve, in the case of a generator like Generator.input, the initial value will wait for a truthy value, if the cell is invalidated, the initial value will never be resolved and the runtime is stuck.

In this change we reject any pending promises when a Generator is disposed, ensuring nothing gets hung. By rejecting we ensure nothing interprets this as an actual value.

I can confirm this fixes a behaviour I was observing when running Notebook 1.0 on Runtime v6.0

[mbostock]

Do we need a similar fix here? Do you mind testing the new standard lib too?

https://github.com/observablehq/notebook-kit/blob/main/src/runtime/stdlib/generators/observe.ts

[tomlarkworthy]

yes the issues exists in Notebook 2.0 too, bit harder to fix with async generator sugar so I converted out.
observablehq/notebook-kit#115
That PR against notebook-kit is not meant to be merged yet, more of a conversation tool but I can can tidy it up if you want.