Insert literal replacement text verbatim when use_regex is off (review SE-2)

re.sub always parses the replacement as a template, so a backslash in
the replacement (e.g. C:\1) raised PatternError even in literal mode.
Also drop an unused import and align js import paths with the
documented two-level extension path. Bump to 2.0.13.

string_multi_replace_node.py

@@ -95,7 +95,10 @@ class StringMultiReplaceNode:
                     f"StringMultiReplace: invalid regex for search '{search_str}': {e}. "
                     f"Disable use_regex to treat search strings as literal text."
                 )
-            result = regex.sub(replace_str, result)
+            # re.sub always parses the replacement as a template (\1, \g<>);
+            # in literal mode insert the user's text verbatim instead.
+            replacement = replace_str if use_regex else replace_str.replace("\\", "\\\\")
+            result = regex.sub(replacement, result)
 
         if remove_extra_spaces:
             result = self.cleanup_text(result)