Use standardx for JS linting

This applies standard for linting JavaScript as per the GDS Way [1]. Standardx
is used instead of standard because Standard 16 introduced a rule
that disallowed the use of `var`, instead preferring `let` or `const` [2].
This conflicts with the GOV.UK approach where we tend to not embrace features
that we know will break old browsers even if they're not necessarily supported
[3], disallowing var will mean that < IE 11 will be unable to run any of the JS.

In order to customise standard rules this project has switched to using
standardx [4] which allows us to disallow rules. I've used this so we
can disallow the 'no-var' rule.

[1]: https://gds-way.cloudapps.digital/manuals/programming-languages/js.html#linting
[2]: standard/standard#633
[3]: alphagov/govuk_publishing_components#1611 (comment)
[4]: https://github.com/standard/standardx

.gitignore

@@ -10,6 +10,7 @@
 /public/finder-frontend/
 /coverage/*
 node_modules
+yarn-error.log
 .env
 credentials.json
-token.yaml
+token.yaml

package.json

@@ -5,10 +5,28 @@
   "author": "Government Digital Service",
   "license": "MIT",
   "scripts": {
-    "lint": "standard app/assets/javascripts/{*.js,**/*.js} spec/javascripts/{*.js,**/*.js}",
-    "lint:fix": "yarn run lint --fix"
+    "lint": "yarn run lint:js",
+    "lint:js": "standardx 'app/assets/javascripts/**/*.js' 'spec/javascripts/**/*.js'"
+  },
+  "standardx": {
+    "env": {
+      "browser": true,
+      "jquery": true,
+      "jasmine": true
+    },
+    "globals": [
+      "GOVUK"
+    ],
+    "ignore": [
+      "spec/javascripts/vendor"
+    ]
+  },
+  "eslintConfig": {
+    "rules": {
+      "no-var": 0
+    }
   },
   "devDependencies": {
-    "standard": "^12.0.1"
+    "standardx": "^7.0.0"
   }
 }