Implemented the CheckURIUseCase use case type in the library target.

Sources/DocCMiddleware/Internal/Extensions/String+Constants.swift

@@ -0,0 +1,20 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the Hummingbird DocC Middleware open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the Hummingbird DocC Middleware project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of Hummingbird DocC Middleware project authors
+//
+// ===----------------------------------------------------------------------===
+
+extension String {
+    /// An empty string.
+    static let empty = ""
+    /// A forwarding slash.
+    static let forwardSlash = "/"
+    /// An indication of a previous folder in a path component.
+    static let previousFolder = ".."
+}