[Setup] Non Apple platforms (#13)

This PR contains the work done to address the issue #12, with regards to provide basic support of non Apple platforms.

To provide further details about the work done:
- [x] flattened the folder structure, especially now that the idea to filter folders based on platform is being discarded;
- [x] implemented precompiler processors to filter out platform-specific source code;
- [x] updated the `Package` file to provide basic support for non-Apple platforms;
- [x] added and also improved some targets to the `Makefile` file to smooth the current development workflows;
- [x] updated the `.gitignore` file with references to the `.vscode` folder and the `.env` file;
- [x] updated the Swift tools version to v5.7.

Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Reviewed-on: #13

Tests/Coordination/Helpers/TestCoordinators.swift

@@ -0,0 +1,112 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftLibs open source project
+//
+// Copyright (c) 2023 Röck+Cöde VoF. and the SwiftLibs project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftLibs project authors
+//
+//===----------------------------------------------------------------------===//
+
+#if canImport(UIKit)
+import Coordination
+import UIKit
+
+// MARK: - Test coordinators
+
+class SomeCoordinator: Coordinator {
+    
+    // MARK: Properties
+    
+    var children: [Coordinator] = []
+    var router: Router
+    
+    // MARK: Initialisers
+    
+    init(router: Router) {
+        self.router = router
+    }
+    
+    // MARK: Functions
+    
+    func present(animated: Bool, onDismiss: (() -> Void)?) {
+        router.present(
+            SomeViewController(),
+            animated: animated,
+            onDismiss: onDismiss
+        )
+    }
+
+}
+
+class SomeOtherCoordinator: Coordinator {
+
+    // MARK: Properties
+    
+    var children: [Coordinator] = []
+    var router: Router
+    
+    // MARK: Initialisers
+    
+    init(router: Router) {
+        self.router = router
+    }
+    
+    // MARK: Functions
+    
+    func present(animated: Bool, onDismiss: (() -> Void)?) {
+        router.present(
+            SomeOtherViewController(),
+            animated: animated,
+            onDismiss: onDismiss
+        )
+    }
+
+}
+
+// MARK: - SpyRouter
+
+class SpyRouter: Router {
+    
+    // MARK: Enumerations
+    
+    enum State {
+        case initialised
+        case presented
+        case dismissed
+    }
+    
+    // MARK: Properties
+    
+    var state: State = .initialised
+    var viewController: UIViewController?
+    var animated: Bool?
+    var onDismiss: OnDismissedClosure?
+    
+    // MARK: Functions
+    
+    func present(
+        _ viewController: UIViewController,
+        animated: Bool,
+        onDismiss: OnDismissedClosure?
+    ) {
+        self.viewController = viewController
+        self.animated = animated
+        self.onDismiss = onDismiss
+        self.state = .presented
+    }
+    
+    func dismiss(animated: Bool) {
+        self.animated = animated
+        self.state = .dismissed
+    }
+    
+}
+
+// MARK: - Test view controllers
+
+class SomeViewController: UIViewController {}
+class SomeOtherViewController: UIViewController {}
+#endif