repl: change var to let

PR-URL: #30428
Reviewed-By: Colin Ihrig <cjihrig@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>

lib/internal/repl/history.js

@@ -41,9 +41,9 @@ function setupHistory(repl, historyPath, ready) {
     }
   }
 
-  var timer = null;
-  var writing = false;
-  var pending = false;
+  let timer = null;
+  let writing = false;
+  let pending = false;
   repl.pause();
   // History files are conventionally not readable by others:
   // https://github.com/nodejs/node/issues/3392