lib: change var to let in string_decoder

PR-URL: #30393
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/string_decoder.js

@@ -57,7 +57,7 @@ function normalizeEncoding(enc) {
 }
 
 const encodingsMap = {};
-for (var i = 0; i < encodings.length; ++i)
+for (let i = 0; i < encodings.length; ++i)
   encodingsMap[encodings[i]] = i;
 
 // StringDecoder provides an interface for efficiently splitting a series of