Run Async Tasks in Series

Implement series(tasks), which runs asynchronous tasks one after another.

tasks is an array of functions. Each takes no arguments and returns a promise or a plain value. series returns a promise that resolves with the array of results, in task order, once the last task has finished.

• A task starts only after the previous task has finished.
• If a task rejects (or throws), stop: the returned promise rejects with that error and the remaining tasks are never started.
• An empty array resolves with [].