Use the core metalsmith postcss plugin

[domoscargin]

Metalsmith is being updated fairly regularly now, and have a core PostCSS plugin.

We've been blocked on bumping some dependencies prior to this (#2016), but are now able to fully bump postcss.

I've run npm build and there is one small prefixing change in the resulting CSS, but otherwise it's exactly the same.

In the generated IE8 CSS we go from:
.govuk-template{background-color:#f3f2f1;-webkit-text-size-adjust:100%;-ms-text-size-adjust:100%;text-size-adjust:100%;overflow-y:scroll}

to:
.govuk-template{background-color:#f3f2f1;-webkit-text-size-adjust:100%;-moz-text-size-adjust:100%;-ms-text-size-adjust:100%;text-size-adjust:100%;overflow-y:scroll}

and in the standard generated CSS we go from:
.govuk-template{background-color:#f3f2f1;-webkit-text-size-adjust:100%;-ms-text-size-adjust:100%;text-size-adjust:100%

to:
.govuk-template{background-color:#f3f2f1;-webkit-text-size-adjust:100%;-moz-text-size-adjust:100%;-ms-text-size-adjust:100%;text-size-adjust:100%

PostCSS major changelogs

8.0.0
7.0.0

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[vanitabarrett]

I wonder if this will unblock #1663 🤔

[domoscargin]

Ooh, interesting. Seems like it probably would.

Edit: Yeah, I did a quick swap for stylelint and looked good (just a bunch of linting failures). Didn't spend long on it, but if I get time I might do a proper PR.

[36degrees]

Changes make sense, but this also bumps metalsmith to v2.5.0 which isn't currently mentioned in the PR or commit message.

Suggest either splitting in to a separate commit or updating the commit message to cover all the changes.

[domoscargin]

Done! metalsmith bump here: #2259

[36degrees]

Thanks for that! I think the changes to the package-lock.json still include Metalsmith 2.5.0 (take a look at the 'rich diff' for the lock file), but suspect you'll need to resolve that once #2259 has been merged as it'll conflict anyway…