swift

255009dd - Implement `#isolation` macro to produce the isolation of the current context

Implement `#isolation` macro to produce the isolation of the current context Introduce a new expression macro that produces an value of type `(any AnyActor)?` that describes the current actor isolation. This isolation will be `nil` in non-isolated code, and refer to either the actor instance of shared global actor in other cases. This is currently behind the experimental feature flag OptionalIsolatedParameters.