Defined the FileServicing protocol in the library target.

Library/Sources/Internal/Protocols/FileServicing.swift

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+/// A type that services access to information from the local file system.
+protocol FileServicing: Sendable {
+    
+    // MARK: Functions
+    
+    /// Lists the names of items located at a given folder. path
+    /// - Parameter folder: A path to a folder that could be found in the local file system.
+    /// - Returns: A list of names related to the items retrieved from a given folder path.
+    /// - Throws: A ``FileServiceError`` error type in case any error happens while retrieving the list.
+    func listItems(in folder: String) async throws (FileServiceError) -> [String]
+    
+}
+
+// MARK: - Errors
+
+/// A representation of all possible errors that could be returned by the type conforming to the ``FileServicing`` protocol.
+enum FileServiceError: Error {
+    /// A given folder path is empty.
+    case folderPathEmpty
+    /// A given folder path is not actually a directory.
+    case folderNotDirectory
+    /// A given folder path has not been found in the file system.
+    case folderNotFound
+    /// Captures any other error found while executing a file service operation.
+    case other(Error)
+}