Migrate from PromiseKit to Swift Concurrency.

This would requite macOS 10.15+, while mas currently supports 10.11+.

This would allow us to upgrade Swift to 5.7+ (Swift 5.7+ through current 6 can run on macOS 10.13+; Swift would, however, limit the macOS versions usable for development to newer versions; see https://developer.apple.com/support/xcode/).

What would we gain & what would we lose (besides support for macOS 10.11-10.14) from this switch?

I have a local branch that I've switched to Swift Concurrency, but I won't submit it until we decide to go ahead with this.

It will be out of date by any time we might decide to make this switch, but it will be at least a helpful starting guide.

See apple/swift-argument-parser#662 (comment) for issues with using Concurrency with SAP & newer versions of Swift / Xcode…