feat(Persistence): SaveSystem implemented and tested

[fingerson]

Implemented the SaveSystem static Class inside the Manager namespace.

Class has 7 static methods:

• file exists static func fileExists(fileName: String) -> Bool
• saveToFile static func saveToFile(content: String, fileName: String) throws
• readFromFile static func readFromFile(fileName: String) throws -> String
• objectFromJson static func objectFromJson(json: String) throws -> T
• objectToJson static func objectToJson(object: T) throws -> String
• saveObject static func saveObject(object: T, fileName: String)
• readObject static func readObject(fileName: String) throws -> T

The class also has its own errors:

enum SaveSystemError: Error { case invalidDirPath case fileDoesNotExist case failedWriting case failedReading case invalidStringEncoding case invalidObject }

Class was tested in a Playground.