Documented the String+Constants extension in the library target.

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

@@ -0,0 +1,8 @@
+extension String {
+    /// A representation of an empty string.
+    static let empty = ""
+    /// A representation of a string containing solely a forwarding slash.
+    static let forwardSlash = "/"
+    /// A representation of a string that identifies a previous folder in a path component.
+    static let previousFolder = ".."
+}