[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/Communications/Cases/Use Cases/MakeURLRequestUseCaseTests.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
 import Communications
 import Foundation
 import XCTest
@@ -141,3 +142,4 @@ private struct TestEndpoint: Endpoint {
     }
 
 }
+#endif