Consider Exposing `cancellationToken` in Signatures

In methods like `getPreEmitDiagnostics` a `CancellationToken` should probably be exposed. I'm running multiple TypeScript jobs in parallel and want to be able to cancel them. I think this would mostly just entail passing through the parameter to the underlying functions.

I'd be willing to contribute this if you'd like.

Start with the getPreEmitDiagnostics signatures and trace the underlying functions they call. Check how TypeScript represents CancellationToken, then identify the related methods that should accept and pass it through; done means callers can cancel parallel TypeScript jobs through the exposed signatures.

Written by the indexing model from the issue text.