Fix tree-shaking when importing from 'govuk-frontend'

[romaricpascal]

What

Ensure that when importing individual components from 'govuk-frontend', only the code from these components get included in the final bundle, and not the entirety of GOV.UK Frontend.

Why

This will make it easier for people to reduce the size of their JavaScript bundle by only including the code of the components they actually use.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• Importing individual components from 'govuk-frontend' includes only the code necessary for these components