Add eslint arrow-spacing check

Author: GuillaumeGomez

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

@@ -38,5 +38,9 @@ module.exports = {
             "error",
             { "before": true, "after": true }
         ],
+        "arrow-spacing": [
+            "error",
+            { "before": true, "after": true }
+        ],
     }
 };