docs(README): replace postcss-cssnext with postcss-preset-env

[halfzebra]

I have replaced the mentions of postcss-cssnext with postcss-preset-env since cssnext is deprecated.

Type

• Docs

SemVer

• Bug (:label: Patch)

Issues

• Fixes Replace cssnext with postcss-preset-env #376

Checklist

• Lint and unit tests pass with my changes
• I have added tests that prove my fix is effective/works
• I have added necessary documentation (if appropriate)
• Any dependent changes are merged and published in downstream modules

[michael-ciniawsky]

@halfzebra Thx 🙇

[halfzebra]

Thanks for merging this, I'm wondering if it would make sense to remove autoprefixer from examples featuring postcss-preset-env, because it has autoprefixer already?

[michael-ciniawsky]

🤔 We could remove all current references to autoprefixer and add a autoprefixing example to the examples section instead with a note/warning for postcss-preset-env users about that matter.

### `Autoprefixing`

\```js
// ...example
\```

> :warning: `postcss-preset-env` already includes the autoprefixer plugin.... and so forth