Make properties in layout attributes subclasses nonisolated

UICollectionViewLayoutAttributes has been isolated to the main actor,
but isEqual(object:) and copy(with:zone) is still non-isolated. Fixed
for now, by making our own properties nonisolated.

Parchment/Classes/PagingBorderLayoutAttributes.swift

@@ -1,8 +1,8 @@
 import UIKit
 
 open class PagingBorderLayoutAttributes: UICollectionViewLayoutAttributes {
-    open var backgroundColor: UIColor?
-    open var insets: UIEdgeInsets = UIEdgeInsets()
+    nonisolated(unsafe) open var backgroundColor: UIColor?
+    nonisolated(unsafe) open var insets: UIEdgeInsets = UIEdgeInsets()
 
     open override func copy(with zone: NSZone? = nil) -> Any {
         let copy = super.copy(with: zone) as! PagingBorderLayoutAttributes

Parchment/Classes/PagingCellLayoutAttributes.swift

@@ -3,7 +3,7 @@ import UIKit
 /// A custom `UICollectionViewLayoutAttributes` subclass that adds a
 /// `progress` property indicating how far the user has scrolled.
 open class PagingCellLayoutAttributes: UICollectionViewLayoutAttributes {
-    open var progress: CGFloat = 0.0
+    nonisolated(unsafe) open var progress: CGFloat = 0.0
 
     open override func copy(with zone: NSZone? = nil) -> Any {
         let copy = super.copy(with: zone) as! PagingCellLayoutAttributes

Parchment/Classes/PagingIndicatorLayoutAttributes.swift

@@ -1,7 +1,7 @@
 import UIKit
 
 open class PagingIndicatorLayoutAttributes: UICollectionViewLayoutAttributes {
-    open var backgroundColor: UIColor?
+    nonisolated(unsafe) open var backgroundColor: UIColor?
 
     open override func copy(with zone: NSZone? = nil) -> Any {
         let copy = super.copy(with: zone) as! PagingIndicatorLayoutAttributes