Move Javascript modules to es6-components

To prevent browsers evaluating JS that isn't supported, components that
contain code from govuk-frontend have been moved to a separate file
`es6-components.js` which is included in `application.html.erb` as a
script tag with `type="module"`.

app/assets/config/manifest.js

@@ -2,6 +2,7 @@
 
 //= link joint.css
 //= link application.js
+//= link es6-components.js
 //= link dagre.js
 //= link joint.layout.DirectedGraph.js
 //= link joint.js

app/assets/javascripts/application.js

@@ -1,9 +1,6 @@
 //= require govuk_publishing_components/lib
-//= require govuk_publishing_components/components/checkboxes
-//= require govuk_publishing_components/components/error-summary
 //= require govuk_publishing_components/components/govspeak
 //= require govuk_publishing_components/components/intervention
 //= require govuk_publishing_components/components/print-link
-//= require govuk_publishing_components/components/radio
 //= require govuk_publishing_components/components/step-by-step-nav
 //= require govuk_publishing_components/components/table

app/assets/javascripts/es6-components.js

@@ -0,0 +1,12 @@
+// These modules from govuk_publishing_components
+// depend on govuk-frontend modules. govuk-frontend
+// now targets browsers that support `type="module"`.
+//
+// To gracefully prevent execution of these scripts
+// on browsers that don't support ES6, this script
+// should be included in a `type="module"` script tag
+// which will ensure they are never loaded.
+
+//= require govuk_publishing_components/components/checkboxes
+//= require govuk_publishing_components/components/error-summary
+//= require govuk_publishing_components/components/radio

app/views/layouts/application.html.erb

@@ -27,6 +27,7 @@
     <%= csp_meta_tag %>
     <%= stylesheet_link_tag "application", :media => "all" %>
     <%= javascript_include_tag "test-dependencies" if Rails.env.test? %>
+    <%= javascript_include_tag "es6-components", type: "module" %>
     <%= javascript_include_tag "application", defer: true %>
     <%= yield :head %>
     <% if content_item %>