Implemented phase 1 for the POC prototype in the Website service.

Services/Website/Sources/Library/Internal/Extensions/String+Routes.swift

@@ -1,3 +1,5 @@
+import Foundation
+
 extension String {
     /// A namespace for the paths of the site's screens.
     ///
@@ -35,5 +37,36 @@ extension String {
         static let terms = "/terms"
         /// The want list.
         static let wants = "/wants"
+
+        /// The path of a listing's detail screen.
+        /// - Parameter slug: the listing's URL slug.
+        static func listing(
+            _ slug: String
+        ) -> String {
+            "/listing/\(slug)"
+        }
+
+        /// The path of a shop's public screen.
+        /// - Parameter slug: the shop's URL slug.
+        static func shop(
+            _ slug: String
+        ) -> String {
+            "/shop/\(slug)"
+        }
+
+        /// The path of the sign-in screen, returning to the given path afterwards.
+        /// - Parameter path: the path the visitor's gated action started from.
+        static func signIn(
+            returningTo path: String
+        ) -> String {
+            "\(signIn)?return=\(path.addingPercentEncoding(withAllowedCharacters: .urlQueryValueAllowed) ?? "")"
+        }
     }
 }
+
+// MARK: - Constants
+
+private extension CharacterSet {
+    /// The characters a query value carries unescaped.
+    static let urlQueryValueAllowed = CharacterSet.urlQueryAllowed.subtracting(.init(charactersIn: "&=+?#"))
+}