Add metadata to Prototype Kit plugin config

[nataliecarey]

We're working on a new plugin page, we'll be making use of new metadata that comes from plugin config. This PR adds that metadata.

I've also added the validator to the post-build tests, this uses npx and @latest to always look at the latest requirements. I recommend this approach but it does erode the purity of repeatable builds, you could hardcode a version number in there or install govuk-prototype-kit as a (dev) dependency and remove both npx and @latest.

I've not added any tests or a strict type definition because I've added the validator which imposes the rules more strictly than any test I'd be contributing here.

[colinrotherham]

Ahh this is brilliant, thank you @nataliecarey

We've got a spike open at the moment which will eventually include GOV.UK Prototype Kit in our automated tests:

• [SPIKE] Run the GOV.UK Prototype Kit locally #4056

Would you mind if I included these changes in that PR?

Once v5 testing is complete we'll switch to managing govuk-prototype-kit versions with Dependabot

[colinrotherham]

Ahh I've tried it with the frontend-v5-compatibility branch and it says:

Error: The following invalid keys exist in your config: constants

So we'll hold off on my last suggestion until constants is the approach for us

But this is still really useful so I've pushed up some config tweaks from the spike too

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-4.6.0.min.css	118.07 KiB
dist/govuk-frontend-4.6.0.min.js	47.1 KiB
dist/govuk-frontend-ie8-4.6.0.min.css	79.27 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	72.34 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	67.95 KiB
packages/govuk-frontend/dist/govuk/all.mjs	4 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	34.28 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.83 KiB

Modules

File	Size
all.mjs	64.5 KiB
components/accordion/accordion.mjs	20.86 KiB
components/button/button.mjs	3.67 KiB
components/character-count/character-count.mjs	20.46 KiB
components/checkboxes/checkboxes.mjs	4.37 KiB
components/error-summary/error-summary.mjs	4.96 KiB
components/exit-this-page/exit-this-page.mjs	15.42 KiB
components/header/header.mjs	2.3 KiB
components/notification-banner/notification-banner.mjs	3.5 KiB
components/radios/radios.mjs	3.37 KiB
components/skip-link/skip-link.mjs	2.12 KiB
components/tabs/tabs.mjs	8.04 KiB

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Action run for 596d962

[nataliecarey]

@colinrotherham looks good to me.