(Async)Iterator support

[jimmed]

I love using listr for handling complex trees of async tasks, and I especially love the way it handles Observables/ReadableStreams by rendering the last emitted value below the task title.

Recently, I've been learning about (and experimenting with) the AsyncIteratable pattern which has been supported in node.js since v10.

While AsyncIterables behave subtly differently to Observables and ReadableStreams, they at least have in common that they asynchronously yield zero or more values before completing.

Accordingly, it would be amazing for listr to accept AsyncIterables as a task callback's return value, much as it already supports Observables and ReadableStreams.