[swift] Add Apple's swift-format

[sureshjoshi]

[sureshjoshi]

This will involve some interesting re-thinking.

Swift operates at the "module" level, so Package.swift is top-level, and then all sources are bundled together and compiled as a module (similar to Go).

[sureshjoshi]

Review this before any Swift support: https://github.com/apple/swift/blob/main/docs/Driver.md

And maybe this to help with dep inference: https://github.com/apple/swift/blob/main/docs/DependencyAnalysis.md