Install node based JS and SCSS linters

This is added to this project as part of roll-out across GOV.UK of stylelint
to replace scss_lint-govuk as the default linter.

This project previously didn't have any JS linting installed so Standard is
installed as per the GDS Way [1]. For SCSS linting stylelint-config-gds [2]
is used which is a GDS standard based upon stylelint [3]. Both of these
projects are then added to dependabot to have their versions maintained with
the same approach we use for rubocop-govuk.

Standardx is used instead of standard because Standard 16 introduced a rule
that disallowed the use of `var`, instead preferring `let` or `const` [4].
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
[5], 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 [6] 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]: https://github.com/alphagov/stylelint-config-gds
[3]: https://stylelint.io/
[4]: standard/standard#633
[5]: alphagov/govuk_publishing_components#1611 (comment)
[6]: https://github.com/standard/standardx

.github/dependabot.yml

@@ -27,6 +27,20 @@ updates:
       - dependency-name: sassc-rails
         dependency-type: direct
 
+  - package-ecosystem: npm
+    directory: /
+    schedule:
+      interval: daily
+    allow:
+      # Internal packages
+      - dependency-name: stylelint-config-gds
+        dependency-type: direct
+      # Framework packages
+      - dependency-name: standardx
+        dependency-type: direct
+      - dependency-name: stylelint
+        dependency-type: direct
+
   # Ruby needs to be upgraded manually in multiple places, so cannot
   # be upgraded by Dependabot. That effectively makes the below
   # config redundant, as ruby is the only updatable thing in the

Jenkinsfile

@@ -3,5 +3,9 @@
 library("govuk")
 
 node {
-  govuk.buildProject(brakeman: true)
+  govuk.buildProject(
+    beforeTest: { sh("yarn install") },
+    brakeman: true,
+    sassLint: false,
+  )
 }

package.json

@@ -0,0 +1,31 @@
+{
+  "name": "calculators",
+  "description": "Frontend application for GOV.UK",
+  "private": true,
+  "author": "Government Digital Service",
+  "license": "MIT",
+  "scripts": {
+    "lint": "yarn run lint:js && yarn run lint:scss",
+    "lint:js": "standardx 'app/assets/javascripts/**/*.js'",
+    "lint:scss": "stylelint app/assets/stylesheets/"
+  },
+  "standardx": {
+    "env": {
+      "browser": true,
+      "jquery": true
+    }
+  },
+  "eslintConfig": {
+    "rules": {
+      "no-var": 0
+    }
+  },
+  "stylelint": {
+    "extends": "stylelint-config-gds/scss"
+  },
+  "devDependencies": {
+    "standardx": "^7.0.0",
+    "stylelint": "^13.8.0",
+    "stylelint-config-gds": "^0.1.0"
+  }
+}