[Library] Foundation library setup (#3)

This PR contains the work done to define a `Libraries` package that includes its `Foundation` library in it, and integrated the former to the `Feed` framework and `App` targets in the project as well.

Reviewed-on: rock-n-code/app-reviews#3
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Libraries/Foundation/Kit/Sources/Extensions/String+Constants.swift

@@ -0,0 +1,18 @@
+//
+//  String+Constants.swift
+//  ReviewsFoundation
+//
+//  Created by Javier Cicchelli on 16/03/2024.
+//  Copyright © 2024 Röck+Cöde VoF. All rights reserved.
+//
+
+import Foundation
+
+public extension String {
+    
+    // MARK: Constants
+    
+    /// A representation of an empty string.
+    static let empty: String = ""
+    
+}

Libraries/Foundation/Test/Tests/Extensions/String+ConstantsTests.swift

@@ -0,0 +1,27 @@
+//
+//  String+ConstantsTests.swift
+//  ReviewsFoundationTest
+//
+//  Created by Javier Cicchelli on 16/03/2024.
+//  Copyright © 2024 Röck+Cöde VoF. All rights reserved.
+//
+
+import ReviewsFoundation
+import XCTest
+
+final class String_ConstantsTests: XCTestCase {
+    
+    // MARK: Properties
+    private var sut: String!
+    
+    // MARK: Constants tests
+    func testEmpty() throws {
+        // GIVEN
+        sut = .empty
+
+        // WHEN
+        // THEN
+        XCTAssertTrue(sut.isEmpty)
+    }
+    
+}