[Pitch] Control default actor isolation inference

[allevato (Tony Allevato)]

@xwu

We could even ape the naming more completely and call the alias DefaultActorIsolationType, and—following the design pitched here for SwiftPM—somehow let users write nil to indicate the nonisolated default (although I'm realizing there may be shenanigans for type inference reasons if we want that).

Spitballing here, but we could define this in the standard library:

public typealias DefaultActionIsolationType = Never

(It's Never isolated by default; i.e., nonisolated.)

And then individual modules could replace it with their own typealias, or they could still write Never in their own code if they wanted to be explicit about the default.