Use type=module for application.js

[AshGDS]

What / Why

• Use type=module for this application's JS file
• To coincide with Drop support for legacy browsers govuk_publishing_components#4111
• Has [DO NOT MERGE] as we want the above PR live on static first before this is merged.
• We will combine es6-components.js again as a separate task unless you disagree with this
• To accurately test this you will need to use local static that is using a local version the govuk_publishing_components branch linked above. If you test without, this app's application.js seems to crash.
• The tests are passing locally with the type=module branch of the gem, so the PR failing is likely due to the deployed version of the gem currently not having type=module as well.
• Trello card: https://trello.com/c/Hoa45SMD/141-turn-off-javascript-in-legacy-browsers

[MartinJJones]

The code changes look to me 👍

The JS error in the browser relates to the closest polyfill, further info here - alphagov/smart-answers#6846 (review)

I'm happy to give this another look and approve when the new version of the gem is released and included in this PR as this should then resolve the issue.