[domain-task] addTask: return promise

[stephtr]

Wouldn't it be more useful if addTask would return the original promise?
One could then write const response = await addTask(fetch('/api/test')); instead of

const task = fetch('/api/test');
addTask(task);
const response = await task;

Edit: @DanHarman also mentioned it.

[SteveSandersonMS]

Thanks - I'll close this in favour of #1252.