[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/Persistence/Helpers/TestEntity.swift

@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import CoreData
+
+@objc(TestEntity)
+public class TestEntity: NSManagedObject {}
+
+// MARK: - Fetch requests
+
+extension TestEntity {
+    
+    // MARK: Functions
+    
+    @nonobjc public class func fetchRequest() -> NSFetchRequest<TestEntity> {
+        return NSFetchRequest<TestEntity>(entityName: "TestEntity")
+    }
+    
+}

Tests/Persistence/Helpers/TestModel.xcdatamodeld/Model.xcdatamodel/contents

@@ -1,4 +0,0 @@
-<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
-<model type="com.apple.IDECoreDataModeler.DataModel" documentVersion="1.0" lastSavedToolsVersion="21754" systemVersion="22E261" minimumToolsVersion="Automatic" sourceLanguage="Swift" userDefinedModelVersionIdentifier="">
-    <entity name="TestEntity" representedClassName="TestEntity" syncable="YES" codeGenerationType="class"/>
-</model>

Tests/Persistence/Helpers/TestPersistenceService.swift

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import CoreData
+import Foundation
 import Persistence
 
 struct TestPersistenceService {