assert: replace var with let in lib/assert.js

Refs: nodejs/code-and-learn#97

PR-URL: #30261
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/assert.js

@@ -186,7 +186,7 @@ function getCode(fd, line, column) {
     buffer = lines < line ? buffer : Buffer.allocUnsafe(bytesPerRead);
     bytesRead = readSync(fd, buffer, 0, bytesPerRead);
     // Read the buffer until the required code line is found.
-    for (var i = 0; i < bytesRead; i++) {
+    for (let i = 0; i < bytesRead; i++) {
       if (buffer[i] === 10 && ++lines === line) {
         // If the end of file is reached, directly parse the code and return.
         if (bytesRead < bytesPerRead) {
@@ -863,7 +863,7 @@ assert.ifError = function ifError(err) {
       tmp2.shift();
       // Filter all frames existing in err.stack.
       let tmp1 = newErr.stack.split('\n');
-      for (var i = 0; i < tmp2.length; i++) {
+      for (let i = 0; i < tmp2.length; i++) {
         // Find the first occurrence of the frame.
         const pos = tmp1.indexOf(tmp2[i]);
         if (pos !== -1) {