Introduced @AtomicArray property wrapper

[sgr-ksmt]

I just implemented @AtomicArray property wrapper.

• Usage

struct Task: Codable {
    var title: String = ""
    @AtomicArray var userIDs: [String] = ["uuuu", "zzzz"]
}

Snapshot<Task>.get(taskPath) { result in
    guard let task = try? result.get() else {
        return 
    }
    
    // replace array
    task.data.userIDs = ["aaa", "bbb"] // => result: ["aaa", "bbb"]
    
    // append element(s) to array
    task.data.$userIDs.union(["xxxx", "yyyy"]) // => result: ["uuuu", "zzzz", "xxxx", "yyyy"]

    // remove element(s) from array
    task.data.$userIDs.remove("zzzz") // => result: ["uuuu"]

    task.update()
}