Swift初始化语法糖库Then

Swift初始化语法糖库Then，使初始化代码更简洁。

使用Then初始化UILabel并设置其属性：

let label = UILabel().then {
  $0.textAlignment = .center
  $0.textColor = .black
  $0.text = "Hello, World!"
}

let label: UILabel = {
  let label = UILabel()
  label.textAlignment = .center
  label.textColor = .black
  label.text = "Hello, World!"
  return label
}()