Add Swift for Xcode related command line tools instead of Go

Some helper tools must inspect or modify Xcode projects, such as reading the project structure, updating asset catalogs, or driving xcodebuild. While ADR-0000008 chose Go for cross platform tools, Go cannot reuse the iOS codebase and must reimplement Xcode-specific formats. Which language should be used for Xcode related command line tools?

Chosen option: "Use Swift", because it shares the language and tooling of the iOS app, reads and writes Xcode project formats naturally, and compiles to a native macOS binary with the already installed Xcode toolchain. Positive consequences: Xcode-specific logic can be prototyped or shared with app code, and no extra toolchain is needed on macOS. Negative consequences: the resulting tools are macOS-only unless the cross platform Swift toolchain is also installed.

Swift is the best option for Xcode related tooling, because it is the native language of Apple platforms and integrates directly with Xcode and xcodebuild. If Go was chosen, Xcode project formats would need to be reimplemented. If Bash scripts were used, the logic would be harder to test and maintain as it grows.