Add support for watchOS 5

[flaw600]

Currently, the String extension requires watchOS 6 to build. For iPhone devices that cannot upgrade to iOS 13, this means that watchOS remains stuck at v5. This requires a complicated setup where a watchOS app has to manually defer to the iPhone via WKSession in order to use Swifter which leads to a more bloated codebase than would otherwise be required for a watchOS app MVP

Offending API usage (restricted to watchOS 6+):

   var queryStringParameters: Dictionary<String, String> {
        var parameters = Dictionary<String, String>()

        let scanner = Scanner(string: self)

        var key: String?
        var value: String?

        while !scanner.isAtEnd {
            key = scanner.scanUpToString("=") //this method is restricted to watchOS 6+
            _ = scanner.scanString(string: "=")

            value = scanner.scanUpToString("&")
            _ = scanner.scanString(string: "&")

            if let key = key, let value = value {
                parameters.updateValue(value, forKey: key)
            }
        }
        
        return parameters
    }

As currently written, the scanUpToString extension in Scanner++.swift does not support watchOS. As a result, dynamic linking tries to use the version from Foundation which requires watchOS 6. For devices that cannot upgrade to watchOS 6, this requires invoking WKSession to manually communicate between iOS & watchOS, which is not ideal