[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/Persistence/Classes/Fetcher.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(Combine) && canImport(CoreData)
 import Combine
 import CoreData
 
@@ -167,3 +168,4 @@ public enum Change: Hashable {
     case section(SectionUpdate)
     case object(ObjectUpdate)
 }
+#endif

Sources/Persistence/Protocols/Service.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if canImport(CoreData)
 import CoreData
 
 public protocol Service {
@@ -38,3 +39,4 @@ public protocol Service {
     func save(childContext context: NSManagedObjectContext) throws 
     
 }
+#endif