[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

Sources/Coordination/Routers/BaseNavigationRouter.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if os(iOS)
 import UIKit
 
 /// This is a base class for the `NavigationRouter` concrete router implementations.
@@ -70,3 +71,4 @@ extension BaseNavigationRouter: UINavigationControllerDelegate {
     }
 
 }
+#endif

Sources/Coordination/Routers/ModalNavigationRouter.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if os(iOS)
 import UIKit
 
 /// This class is responsible for showing view controllers modally, as it is a concrete implementation of the `Router` protocol.
@@ -88,3 +89,4 @@ private extension ModalNavigationRouter {
     }
     
 }
+#endif

Sources/Coordination/Routers/PushNavigationRouter.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if os(iOS)
 import UIKit
 
 /// This class is responsible for pushing view controllers into a navigation controller, as it is a concrete implementation of the `Router` protocol.
@@ -66,3 +67,4 @@ extension PushNavigationRouter: Router {
     }
     
 }
+#endif

Sources/Coordination/Routers/WindowRouter.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if os(iOS)
 import UIKit
 
 /// This class is responsible for populating the window of an application.
@@ -45,3 +46,4 @@ public class WindowRouter: Router {
     }
     
 }
+#endif