Stabilize abort signal in retry

[tomas-zijdemans]

I'd like to propose stabilizing the support for abort signal in retry.ts. I think the rationale is quite simple:

• It makes retry() compose well with other async functions (delay, deadline, abortable, circuit_breaker)
• It's extra handy in the browser where you often need to cancel in-flight operations when a user navigates away or a component unmounts
• It's a straightforward and backwards compatible change ☺️

Here is a PR that implements it.