Implemented the FileService service in the library target.

Library/Sources/Internal/Services/FileService.swift

@@ -0,0 +1,31 @@
+import Foundation
+
+struct FileService: FileServicing {
+    
+    // MARK: Functions
+    
+    func listItems(in folder: String) async throws (FileServiceError) -> [String] {
+        let fileManager = FileManager.default
+        
+        guard !folder.isEmpty else {
+            throw .folderPathEmpty
+        }
+        
+        var isFolder: ObjCBool = false
+        
+        guard fileManager.fileExists(atPath: folder, isDirectory: &isFolder) else {
+            throw .folderNotFound
+        }
+        
+        guard isFolder.boolValue else {
+            throw .folderNotDirectory
+        }
+
+        do {
+            return try fileManager.contentsOfDirectory(atPath: folder)
+        } catch {
+            throw .other(error)
+        }
+    }
+    
+}