Don't include unavailable setter in Swift 5.9.

[mbrandonw]

Addresses #63.

There seems to be a bug in Swift <=5.10 where the unavailable setter on Shared.wrappedValue makes even the getter unavailable when using dynamic member lookup with subscripts:

// Swift <=5.10
func foo() -> Int {
  @Shared(.inMemory("favorites")) var favorites = [42]
  return favorites[0]  // 🛑 Unavailable setter
}

You can reproduce this with any subscript that has a setter. The solution is to just omit the unavailable setter in Swift <=5.10.

This bug is still present in Swift 6, but to a lesser degree. If you access through the subscript without assign, Swift thinks we are invoking the setter somehow:

// Swift >=6
func foo() {
  @Shared(.inMemory("favorites")) var favorites = [42]
  favorites[0]  // 🛑 Unavailable setter
}

But since there is no real world use case for doing something like this I think we can live with it.

[mbrandonw]

It's not possible to surround just the set { … } in the #if check. At least not in Swift 5.10.