Update Review app to import govuk-frontend via local package

[colinrotherham]

This PR makes the Review app a user of govuk-frontend as part of:

• Gather built files sizes and display them in review app #3280

For the performance work all source code and build artifacts can now be resolved from a single package

Additionally, the Review app now follows our own documentation:
https://frontend.design-system.service.gov.uk/importing-css-assets-and-javascript/

These areas in particular:

Import CSS, assets and JavaScript

1. Simplify Sass import paths
2. Import JavaScript using a bundler

Which means the Review app now imports govuk-frontend ESM JavaScript for the first time, without having to navigate to ../../../src sources or rely on a separate review-specific UMD bundle

Package directory changes

Previously npm link only resolved the pre-built packages files but they're now consolidated:

Before

├── app
│   ├── src
│   └── dist
└── package
    ├── govuk
    ├── govuk-esm
    ├── govuk-prototype-kit
    ├── package.json
    └── README.md

After

└── packages
    ├── govuk-frontend
    │   ├── src
    │   │   ├── govuk
    │   │   └── govuk-prototype-kit
    │   ├── dist
    │   │   ├── govuk
    │   │   ├── govuk-esm
    │   │   └── govuk-prototype-kit
    │   ├── package.json
    │   └── README.md
    └── govuk-frontend-review
        ├── src
        └── dist

src → packages/govuk-frontend/src
package → packages/govuk-frontendpackage/dist

To maintain backwards compatibility these directory changes aren't found in the published package.json

The problem with dist

Unfortunately the standard convention of building src to dist doesn't fit

We already reserve ./dist for our GitHub tagged releases, do we need to move it?
Will it be confusing to have a committed ./dist alongside ./package/src → ./package/dist build output?

Some options:

1. dist — Leave it where it is
2. dist → package/release — Maintain two build directories under ./package
3. dist → package/dist/release — Consolidate single build directory under ./package

This PR has currently picked 1) from the list above

[domoscargin]

I think this looks great - thanks for breaking up the PRs and keeping everything in sync, @colinrotherham, it's been really helpful.

Would appreciate another pair of eyes on this before merging, but from our discussion it didn't seem like there'd be any major objections.

Probably also a heads up to the team in Slack when this merges would be good, just to flag that folks are probably going to need to do some rebasing and such.

[domoscargin]

Also worth popping a reminder here that this PR means we have to update our Decision on repo structure: alphagov/govuk-design-system-architecture#24

(I see @36degrees has just added that to the board as well)

[romaricpascal]

Looks good to go for me as well, it's all super tidy ⛵ Thanks for keeping with all the feedback and rebases 😊

[romaricpascal]

Agree with Brett on notifying people before we merge it, and making sure we don't wait too long before rebasing the PRs that still need rebasing as well 😊

[colinrotherham]

Agree with Brett on notifying people before we merge it, and making sure we don't wait too long before rebasing the PRs that still need rebasing as well 😊

Thanks @romaricpascal

We rebased all these earlier:

cssnano-autoprefixer (#3243)
delete-package-dist (#3498)
remove-compat-mode-legacy-colours (#3576)
remove-compat-mode-legacy-fonts (#3574)
remove-ie8-css (#3572)
remove-ie8-js (#3570)
remove-typography-use-rem-setting (#3575)