Add support for Swift 6

[bbrk24]

Checklist

• I searched the existing GitHub issues and this hasn't already been requested.

Problem

Currently, Dependiject always compiles itself as Swift 5. Xcode 16 now gives warnings if you're compiling in Swift 5 mode and something would be an error in Swift 6.

Feature Request

Dependiject should support both Swift 5 and Swift 6.

Value

Reduces warnings for downstream consumers who are using Xcode 16.

[bbrk24]

There's more to consider here than just the concurrency warnings/errors, e.g. BitwiseCopyable.