nixpkgs.lib: Add escapeNixString

lib/strings.nix

···
   */
   escapeShellArgs = concatMapStringsSep " " escapeShellArg;
 
+  /* Turn a string into a Nix expression representing that string
+
+     Example:
+       escapeNixString "hello\${}\n"
+       => "\"hello\\\${}\\n\""
+  */
+  escapeNixString = s: escape ["$"] (builtins.toJSON s);
+
   /* Obsolete - use replaceStrings instead. */
   replaceChars = builtins.replaceStrings or (
     del: new: s: