Use nesting in CSS

[jeddy3]

Because:

• CSS nesting is being picked up as an Editor's Draft
• we already use features like custom properties and color-mod that need transpiling
• postcss-preset-env supports it out-of-the-box

The scope of spec is narrower than the nesting in preprocessors like Sass. We can use stylelint to limit nesting to direct nesting with the explicit & (i.e. disallows @nest for now).

To be nest-prefixed, a nesting selector must be the first simple selector in the first compound selector of the selector.

It feels like it'd be great for pseudo-classes and the like. Improving the readability of the code without opening the can of worms that can be nesting.