readline: change var to let

PR-URL: #30435
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

Author: dnlup

lib/internal/readline/utils.js

@@ -16,7 +16,7 @@ let isFullWidthCodePoint;
 
 function CSI(strings, ...args) {
   let ret = `${kEscape}[`;
-  for (var n = 0; n < strings.length; n++) {
+  for (let n = 0; n < strings.length; n++) {
     ret += strings[n];
     if (n < args.length)
       ret += args[n];
@@ -79,7 +79,7 @@ if (internalBinding('config').hasIntl) {
 
     str = stripVTControlCharacters(String(str));
 
-    for (var i = 0; i < str.length; i++) {
+    for (let i = 0; i < str.length; i++) {
       const code = str.codePointAt(i);
 
       if (code >= kUTF16SurrogateThreshold) { // Surrogates.