[Request] Improve cancellation API

Is your feature request related to a problem? Please describe. We've gotten feedback that the Symbol('clack:cancel') approach and the onCancel don't feel like the ideal API for handling cancellation, and I agree.

I'm opening this issue to continue the discussion about what an API for graceful cancellation could look like.

Describe the solution you'd like Seems like AbortController is designed specifically for this use case, so maybe there's some inspiration to take from the signal pattern?

Describe alternatives you've considered Open to discussion!

Additional context See https://github.com/natemoo-re/clack/pull/68