[linter] Add rule no-var to disallow the use of var in favor of let and const.

ivette/.eslintrc.js

@@ -77,5 +77,7 @@ module.exports = {
     "react/destructuring-assignment": "off",
     // Allow console errors and warnings
     "no-console": ["error", { allow: ["warn", "error"] }],
+    // Disallow the use of var in favor of let and const
+    "no-var": "error",
   }
 };