Skip Lite: Add support for `.defaultIsolation(.mainActor)`

[dfabulich]

Swift 6.2 allows targets to opt into marking classes as @MainActor by default. https://github.com/swiftlang/swift-evolution/blob/main/proposals/0466-control-default-actor-isolation.md

SwiftPM packages can opt-in to the behavior like this:

.target(
    name: "MyLibrary",
    swiftSettings: [
        .defaultIsolation(.mainActor), // Sets the whole module to @MainActor
    ]
)

Skip Lite doesn't know anything about that. (The SwiftPM build plugin API doesn't even expose swiftSettings.)

We'd presumably need to add a skip.yml setting to enable .defaultIsolation(.mainActor), and then the transpiler would need to be updated to support that.

Apple's guidance is that apps/UI libraries should opt-in to .defaultIsolation(.mainActor), but non-UI libraries should opt out. I think it would make sense to have skip create prompt the user whether they want to use .defaultIsolation(.mainActor), defaulting to "yes" for apps, and "no" for libraries.

[dfabulich]

Actually, bet we could swift package dump-package to extract the settings instead of skip.yml... 🤔

[marcprux]

Actually, bet we could swift package dump-package to extract the settings instead of skip.yml... 🤔

I think swift package dump-package will try to connect to the network to resolve the packages, and that will be blocked by the sandbox that SwiftPM runs plugins in (which blocks both network access as well as writing outside of the project writable directory).

[dfabulich]

Ah, well. Maybe skip verify could do it, at least.