[Enhancement] String localisation (#21)

This PR contains the work done to provide string localisation support for the `Core` library.

To provide further details about the work done:
- [x] implemented the `localisation(for:)` function for the `Bundle+LocalisationBundle` extension;
- [x] implemented the `localise(for: in: value: table:)` function for the `String+Localisation` extension;
- [x] defined English as the default localisation language in the `Package.swift` file;
- [x] added the "vscode" target to the `Makefile` file.

Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Reviewed-on: #21

Tests/Core/Resources/en.lproj/Localizable.strings

@@ -0,0 +1,14 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+"test.core.bundle.some-localisable-string" = "Some localisable string to use for testing purposes.";
+"test.core.bundle.other-localisable-string" = "Other localisable string to use for testing purposes.";