Add "no-unused-vars" eslint rule

Author: GuillaumeGomez

src/librustdoc/html/static/.eslintrc.js

@@ -55,5 +55,12 @@ module.exports = {
         "max-len": ["error", { "code": 100, "tabWidth": 4 }],
         "eol-last": ["error", "always"],
         "arrow-parens": ["error", "as-needed"],
+        "no-unused-vars": [
+            "error",
+            {
+                "argsIgnorePattern": "^_",
+                "varsIgnorePattern": "^_"
+            }
+        ],
     }
 };