Release v4.0.0 of GOV.UK Frontend to npm 🚀

[vanitabarrett]

Release GOV.UK Frontend v4.0.0, following our release process.

Done when:

This will be done when:

• the package has been published on npm
• the release has been created on GitHub
• the release has been communicated via our comms channels
• the sprint board has been updated

[lfdebrux]

We may want to change our release process slightly before doing this (see https://github.com/alphagov/design-system-team-internal/issues/542).

[vanitabarrett]

Update: we've decided not to test the new release process (with automation token). Instead, we're going to follow the normal release process. See https://github.com/alphagov/design-system-team-internal/issues/542#issuecomment-987944359 for more details.