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, this app's application.js seems to crash.
• Trello card: https://trello.com/c/Hoa45SMD/141-turn-off-javascript-in-legacy-browsers

[MartinJJones]

Looks good to me, the JS error appears to be caused by the closest polyfill imported from the publishing components gem and using type="module".

If this PR could also include the upgrade to the gem that removes support for older browsers and polyfills when released, this will resolve the issue.

Further info on the error:

Uncaught TypeError:
Cannot read properties of undefined (reading 'Element')
    at application.js:3:6

The reason for this appears to be that a top level this is undefined when used in a module, before this change it was the window object - https://exploringjs.com/es6/ch_modules.html#sec_modules-vs-scripts