[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/Date.swift

@@ -280,6 +280,7 @@ extension Date : CustomPlaygroundQuickLookable {
         return df.string(from: self)
     }
     
+    @available(*, deprecated, message: "Date.customPlaygroundQuickLook will be removed in a future Swift version")
     public var customPlaygroundQuickLook: PlaygroundQuickLook {
         return .text(summary)
     }