How to make simple NSItemProvider in Swift

Issue #728

NSItemProvider(object: StringProvider(string: string))

class StringProvider: NSObject, NSItemProviderWriting {
    let string: String
    init(string: String) {
        self.string = string
        super.init()
    }

    static var writableTypeIdentifiersForItemProvider: [String] {
        return [(kUTTypeData) as String]
    }

    func loadData(
        withTypeIdentifier typeIdentifier: String,
        forItemProviderCompletionHandler completionHandler: @escaping (Data?, Error?) -> Void
    ) -> Progress? {
        let data = string.data(using: .utf8)
        completionHandler(data, nil)
        return Progress(totalUnitCount: 100)
    }
}