Feature Request: Add Async Completion Source

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>` 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.