Locate GOV.UK Frontend using require.resolve()

[colinrotherham]

This PR ensures we locate all previously hard-coded GOV.UK Frontend paths

This is in preparation for path changes between v4 (current) and v5 (upcoming) releases:

GOV.UK Frontend v4

1. Base path: node_modules/govuk-frontend
2. Entry path: node_modules/govuk-frontend/govuk/all.js
3. Include path: node_modules/govuk-frontend/govuk

GOV.UK Frontend v5

1. Base path: node_modules/govuk-frontend
2. Entry path: node_modules/govuk-frontend/dist/govuk/all.bundle.js
3. Include path: node_modules/govuk-frontend/dist/govuk

Partially closes #2293 and resolves alphagov/govuk-frontend#3759

[colinrotherham]

I've moved the GOV.UK Frontend v5 preview commits to another branch

Installing the preview from GitHub via npm install was causing test issues:

npm install --save "alphagov/govuk-frontend#6d1a4ef0a"

Not sure if a few sleep() or cy.wait() commands were expecting it to take so long?

Test output is looking a lot better now with some new changes pushed

[colinrotherham]

Pushed again with the GOV.UK Frontend v5 preview

[colinrotherham]

@nataliecarey I think these lines need a suggestion

As written, Nunjucks views will get govukFrontend set to the internal GOV.UK Frontend version

Is there a router/middleware approach we can use to set:

1. Set Nunjucks local govukFrontend to "internal version" by default
2. Set Nunjucks local govukFrontend to "plugin version" in plugin pages only

[colinrotherham]

Turns out Nunjucks, Sass and the error server had overlapping GOV.UK Frontend versions:

• Fix error server /manage-prototype/dependencies #2333

But now they're separated I've added separate locals for each:

// Add GOV.UK Frontend paths to Nunjucks locals
app.locals.govukFrontend = govukFrontend
app.locals.govukFrontendInternal = govukFrontendInternal

This allows prototype pages and management pages to clearly use their own versions:

{%- set assetPath = '/manage-prototype/dependencies/govuk-frontend' + govukFrontendInternal.assetPath -%}

Or for govuk-branded.njk it can be overridden or default to the plugin version:

{%- set assetPath = assetPath | default('/plugin-assets/govuk-frontend' + govukFrontend.assetPath) -%}

[colinrotherham]

@BenSurgisonGDS has suggested a new getScriptsAndAssetsConfig() utility for this

Couldn't find a "sync" way using an existing helper so might be a better option

[nataliecarey]

Looks good to me, just needs a changelog entry.