Replace light-dark @mixin with native light-dark image support

light-dark() supports images now - which is the only thing we're using postcss-mixins for.

We should therefore be able to replace our uses of @mixin light-dark with native light-dark(), and remove the postcss plugin, and stylelint overrides (e.g. https://github.com/mdn/fred/pull/1861)

However, currently the postcss-light-dark plugin adds a @supports (color: light-dark(red, red)) block which would expose light-dark(<image>, <image>) on browsers which don't support it.

Setting preserve: false on this plugin will allow us to make the switch now, until postcss-light-dark detects image support and uses an appropriate @support block.

We also need to force the postcss-light-dark plugin on, until upstream supports light-dark(<image>, <image>), otherwise it'll stop transpiling once light-dark(<colour>, <colour>) is Widely Available (and we drop support for Firefox 115).

Locate the @mixin light-dark uses, the postcss-mixins and postcss-light-dark configuration, and the stylelint overrides referenced in the issue. Check the existing CSS build and lint checks first. Done means native light-dark() handles images, postcss-mixins and unnecessary overrides are removed, and postcss-light-dark uses preserve: false and remains forced on for image support.