Add support for Swift macros

[antoniostrijdom]

Swift 5.9 introduced Swift macros and, with it, changes to the Package.swift file.

A new .macro target was added. It would be great if we could upgrade the swift-tools-version to 5.9 and add support for defining an optional macro target in <package-name>.json.

This would only be needed once Xcode 15 ships.