Add "no-alert" eslint rule

Author: GuillaumeGomez

Diff for: src/librustdoc/html/static/.eslintrc.js

@@ -77,5 +77,6 @@ module.exports = {
         "no-template-curly-in-string": "error",
         "block-scoped-var": "error",
         "guard-for-in": "error",
+        "no-alert": "error",
     }
 };