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.

test_string_nodes.py

@@ -106,6 +106,20 @@ class TestStringNodes(unittest.TestCase):
         )[0]
         self.assertEqual(result, "Hello everyone")
 
+    def test_replace_literal_backslash_replacement(self):
+        # With use_regex disabled, replacement text is literal: backslash
+        # sequences must not be treated as re.sub group references.
+        result = self.replace_node.string_replace(
+            input_string="path is X",
+            replacement_pairs=r"X::C:\1",
+            replacement_delimiter="::",
+            match_case=False,
+            match_whole_string=True,
+            preserve_punctuation=True,
+            remove_extra_spaces=False
+        )[0]
+        self.assertEqual(result, r"path is C:\1")
+
     def test_replace_multiple_pairs(self):
         input_string = "The quick brown fox jumps over the lazy dog"
         replacement_pairs = """quick::slow