A mutable variant of Binding.constant to help with swiftui previews

Binding+initially.swift

extension Binding {
  /// like Binding.constant but lets you update the value and stores that updated value via a captured enclosing reference.
  public static func initially(_ value: Value) -> Binding<Value> {
    let box = Box(value)
    return .init(
      get: { box.value },
      set: { box.value = $0 }
    )
  }

  private class Box<Value> {
    var value: Value
    init(_ value: Value) { self.value = value }
  }
}