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_conditional_append_node.py

@@ -1,5 +1,3 @@
-import re
-
 class StringConditionalAppendNode:
     @classmethod
     def INPUT_TYPES(cls):