tracing tla promises that are never resolved

[Gus Caplan]

In Node.js we'd like to provide info about TLA stalling due to unresolved promises, and at the moment we just check if the main import promise has resolved by the time the process exits and set the error code. This doesn't tell you where the stall happened though (which `await`). I'm thinking maybe some API could be added like GetStalledSuspendPoint() which optionally returns a Message or something similar so we could print out the source and line and column and whatnot.