Track Swift 6.0 release; Add Swift 6.0 support once released

[alolita]

See more about Swift 6.0 features - https://github.com/swiftlang/swift/blob/main/CHANGELOG.md

Swift 6 support features that would be useful include:

• Existential support / Class hierarchies
• Data race safety
• async/await, actor

You can enable the Swift 6 language mode using the -swift-version 6 compiler flag.