[pigeon][swift 6]use typed throw for swift codegen

Swift 6 introduces ["typed throws"](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0413-typed-throws.md):

```
func foo() throws(MyError) { … }

do {
try foo()
} catch {
// error here is of type MyError (rather than of type `Error`)
}
```

A good way to force developers to throw `PigeonError`.

Only available for Swift 6 though, so will need to fix all the concurrency warnings before we can use it.