[string] Add a fast path to String.p.replace

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 947198b9caba452be8f2102519ae4849c7e15b2f..80530c36ab392b00360d2ede86dd6042e49a627b 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8203,8 +8203,11 @@ class String: public Name {
   // GetSubstitution(matched, str, position, captures, replacement)
   // Expand the $-expressions in the string and return a new string with
   // the result.
+  // A {start_index} can be passed to specify where to start scanning the
+  // replacement string.
   MUST_USE_RESULT static MaybeHandle<String> GetSubstitution(
-      Isolate* isolate, Match* match, Handle<String> replacement);
+      Isolate* isolate, Match* match, Handle<String> replacement,
+      int start_index = 0);
 
   // String equality operations.
   inline bool Equals(String* other);