Compile GOV.UK Prototype Kit config to package top level

[colinrotherham]

This PR moves our GOV.UK Frontend Prototype Kit config for backwards compatibility

1. Ensures config is exported from govuk-frontend/govuk-prototype-kit.config.json
2. Ensures config macros, assets and Nunjucks paths add dist/ prefix

This was previously noted under Restructure govuk-frontend repository for v5 Implications

• We need to keep making sure our configuration for the GOV.UK Prototype Kit gets recognised by the Kit and picks up our files as necessary.

[romaricpascal]

Things worked fined with npm link locally. However, the Prototype Kit config doesn't seem to be pushed to pre-release branches 😔 I've created a pre-release for this PR there: https://github.com/alphagov/govuk-frontend/tree/pre-release-move-config-prototype-kit 😊

[colinrotherham]

Things worked fined with npm link locally. However, the Prototype Kit config doesn't seem to be pushed to pre-release branches 😔 I've created a pre-release for this PR there: https://github.com/alphagov/govuk-frontend/tree/pre-release-move-config-prototype-kit 😊

Sadly we do two things which only #3498 can unblock

1. Pre-releases check for unstaged changes

if [[ -n $(git status --porcelain) ]]; then

2. Pre-releases exclude anything outside packages/govuk-frontend/dist

git filter-branch --force --subdirectory-filter packages/govuk-frontend

I've rebased #3498 with this branch so let's test there?

[romaricpascal]

Tests with #3498 were OK, so let's merge them both one after the other (rather than going through some tedious rebasing).

[colinrotherham]

This ensures GOV.UK Prototype Kit can find the config

But paths at their end are still hard coded: alphagov/govuk-prototype-kit@953830a