Feature Request: Add Async Completion Source

[abbottdev]

It would be nice if we could use an async completion source so that when making expensive calls to evaluate completions, a Ctrl+C escape can signal to the cancellation token to quit.

Proposal: Add AsyncCompletionSource which implements ICompletionSource but uses a Task<IEnumerable<TOption/TArgument>> with a cancellation token that can be signalled via Ctrl+C.

It looks as though the completion shim would need to pass this along somehow - I'd be happy to contribute this if it's deemed in scope and given some pointers around the code flow.