Added a draft introduction for the Hybrid Signal handler

[Arfey]

Introduce a draft proposal for the Hybrid Signal handler, which allows defining separate sync and async implementations for signal receivers to improve performance and simplify mixed context execution.

[carltongibson]

I think the basic idea here is correct @Arfey here: No-reason at all why built-in signals, and those from third-party apps, shouldn't have a way to opt-in to providing both sync and async versions of the same signal handler. 👍