[stdlib] Deprecated PlaygroundQuickLook and CustomPlaygroundQuickLookable.

Deprecated the `PlaygroundQuickLook` enum and `CustomPlaygroundQuickLookable`
protocol. These are being targeted for removal in Swift 5, so we want to
unconditionally deprecate them now to encourage use of
`CustomPlaygroundDisplayConvertible` instead.

This commit includes deprecated the various `CustomPlaygroundQuickLookable`
conformances across the standard library and overlay libraries.

stdlib/public/SDK/Foundation/NSString.swift

@@ -113,6 +113,7 @@ extension NSString {
 }
 
 extension NSString : CustomPlaygroundQuickLookable {
+  @available(*, deprecated, message: "NSString.customPlaygroundQuickLook will be removed in a future Swift version")
   public var customPlaygroundQuickLook: PlaygroundQuickLook {
     return .text(self as String)
   }