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 failing tests will likely be caused by the JS error relating to the closest polyfill, further info here - alphagov/smart-answers#6846 (review)

I'm happy to give this another look when the new version of the gem is released and included in this PR