Drop gyb in favor of swift macros

[Amzd]

We can use swift macros to generate the code that's currently generated by gyb.

Pros

• Code completion and type checking on types that will get generated (rn they are gyb files which get nothing)
• No extra step to run when editing types that have generation step, just swift build.
• No dependency on gyb

Cons

• Requires Swift 5.9 so we will have to up the requirement from currently 5.5

Should be fun project for anyone with decent knowledge of Swift who wants to learn about macros.

[DivineDominion]

Requires Swift 5.9 so we will have to up the requirement from currently 5.5

Swift 5.10 is available on all platforms and runs where 5.5 runs. So there should be no blockers. Swift 6.x is available, too.

(Xcode versions being tied to Swift versions, and Xcode versions being tied to macOS versions could be a factor, but I guess that's not relevant. E.g. the latest Xcode 16.3 requires macOS Sequioa, so folks on Sonoma and lower need to install the latest Swift toolchain manually. But for cross-platform dev, that should not be a breaker?)

So I believe there's no blocker to use macros -- developer time to work on that transition permitting :)

[stackotter]

And since the creation of this issue, SwiftCrossUI has already been bumped to Swift 5.10 😉 I won't be working on this any time soon due to other more pressing issues, but yes if anyone wants to, feel free!