benchmark: add no-var rule in .eslintrc.yaml

PR-URL: #31794
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

benchmark/.eslintrc.yaml

@@ -5,6 +5,7 @@ env:
   es6: true
 
 rules:
+  no-var: error
   comma-dangle:
     - error
     - arrays: 'always-multiline'