Allows the Pattern Library to be used with v5 of the Design System, and
uses v5 by default in the documentation.

This currently continues to support IE8 by providing a copy of the
`govuk-if-ie8` mixin.

This is how the GOV.UK Design System recommends installing v5+.
Doing so also ensures that we can support ES modules for our users, and
highlighted an issue with an undefined variable in the Password reveal JavaScript.

Directly requiring individual govuk-frontend SCSS files is risky, as govuk-frontend could
move their internal files and break our compatibility. This has happened in v5 of
govuk-frontend, and we cannot support both ≤v4 and v5 at the same time with the current
method.

Instead, we will require users to separately import govuk-frontend before moj-frontend. In
reality users should already be doing this, as using moj-frontend code without govuk-frontend
will cause an inconsistent UI. Anyone who _isn't_ doing so already will just need a one line
import:

```scss
@import "node_modules/govuk-frontend/dist/govuk/all";
```

To still be able to build our distributable version, which also requires govuk-frontend code
to be built, create a wrapper `dist.scss` with the specific imports required.

BREAKING CHANGE: If you are building moj-frontend from SCSS, you must include govuk-frontend
beforehand

It's the current LTS so we should be using it in tests

Switch to v5.0.0 of `govuk-frontend` now that it has been released

To give people an easier upgrade path rather than needing to do a paired upgrade.

BREAKING CHANGE: if you build your CSS from this package's SCSS, you may need to update your import
path

Stop building IE8 CSS distributions. Remove IE8-specific rules from SCSS.

BREAKING CHANGE: This package no longer provides support for IE8

To v2.0.0