Defer prototype `application.js` to load after plugin scripts

[colinrotherham]

This PR switches application.js to <script type"module"> to fix:

• https://github.com/alphagov/govuk-prototype-kit/issues/2353

This change defers script load by default and also ensures:

1. ES module import { Accordion } from syntax support in GOV.UK Frontend v5
2. Load order compatibility with other plugins that update to ES modules

Browser support

This follows GOV.UK Frontend to protect browsers without ES6 modules support from script errors, such as:

• Internet Explorer 11
• Edge 15
• Chrome 60
• Firefox 59
• Safari 9

[colinrotherham]

Moved the CHANGELOG entry into Unreleased