Update extensions config to work with Prototype Kit v13

[36degrees]

What

Update the extensions config file and add any additional integration points required for GOV.UK Frontend to work correctly in (the future) v13 of the Prototype Kit.

Why

A new major version of the Prototype Kit will shortly be released which includes some changes to the extension system that affect us.

Specifically, the kit will no longer:

• import the Nunjucks macros from GOV.UK Frontend
• configure asset paths, enable the new link styles or turn on global styles
• call window.GOVUKFrontend.initAll

So, we now need to provide the integration points to take care of those things, some of which relies on new functionality in v13 of the Prototype Kit – some of which may not yet be stable. This means we'll need to work closely with the Prototype Kit team to ensure that what we end up releasing will work with v13.0.0 of the Prototype Kit.

Who needs to work on this

Developers

Who needs to review this

Developers, Developers from the Prototype Kit team

Done when

• We've confirmed what the stable 'API' for the Prototype Kit extensions system will look like
• The extensions config and supporting files, when included in the Prototype Kit, takes care of:
  • importing the Nunjucks macros for all components
  • configuring the asset paths
  • enabling the new link styles
  • turning on global styles
  • calling window.GOVUKFrontend.initAll
• We've shared a pre-release of GOV.UK Frontend with the Prototype team and they have confirmed that it works as expected

[36degrees]

@nataliecarey is open to pairing on this.

She has already raised a PR that could be updated (#2851)

[36degrees]

How should we record this change in the changelog and release notes, if at all?

Arguably it's a new feature but it doesn't seem relevant for folks reading the release notes as it's more about unlocking a feature of the Prototype Kit?

Maybe we should include it in fixes? 'Add support for Prototype Kit v13 plugin system'?

[colinrotherham]

Reopening until changelog and release notes added

[colinrotherham]

Just included it under "Fixes" as discussed: #2981