Add SwiftUI Property declaration to sub section ordering list

[miguel-jimenez-0529]

Summary

After merging nicklockwood/SwiftFormat#1747 and #280, this PR adds the SwiftUI property declaration ordering rule to the style guide.

Reasoning

SwiftUI Properties are a special type of property that lives inside SwiftUI views. These views conform to the DynamicProperty protocol and cause the a view's body to re-compute. Given this common functionality and also a similar syntax, it is preferred to group them together.

// WRONG

struct CustomSlider: View {

  // MARK: Internal

  var body: some View {
    ...
  }

  // MARK: Private

  @Binding private var value: Value
  private let range: ClosedRange<Double>
  @Environment(\.sliderStyle) private var style
  private let step: Double.Stride
  @Environment(\.layoutDirection) private var layoutDirection
}

// RIGHT

  struct CustomSlider: View {
  
  // MARK: Internal

  var body: some View {
    ...
  }

  // MARK: Private

  @Environment(\.sliderStyle) private var style
  @Environment(\.layoutDirection) private var layoutDirection
  @Binding private var value: Value
  
  private let range: ClosedRange<Double>
  private let step: Double.Stride
}

[calda]

Nice!

[bachand]

@miguel-jimenez-0529 what do you think about generalizing this section to be any property with a property wrapper? I'm curious about the reason to scope this specifically to SwiftUI property wrappers. cc: @calda

[miguel-jimenez-0529]

Me and Cal had the same conversation. Our conclusion was that there are @ properties like @objc that are not property wrappers. Also a property wrapper doesn't have a common semantic meaning, examples of those are @Referenced, @UniBindable, @Atomic, @Resilient. So grouping all property wrappers when we don't know upfront their functionality was a bit opinionated. Engs may have a better idea on where to place them.
This is different for SwiftUI property wrappers since they all conform to the DynamicProperty protocol and have a common semantic meaning in the sense that they interact with the UI.

[bachand]

This makes sense. Thanks for explaining!

[calda]

Thanks @miguel-jimenez-0529! Lots of support for this change.