Apply prefer-const lint rule

lib/config.js

@@ -138,7 +138,7 @@ function setConfig(name, value, commandLineArgument) {
   }
 
   let _value = null;
-  for (let [optionName, optionDetails] of Object.entries(options)) {
+  for (const [optionName, optionDetails] of Object.entries(options)) {
     let n = optionName;
     if (commandLineArgument) n = n.toLowerCase().replace(/_/g, "-");