Add PostCSS import plugin as default

[pinzonjulian]

https://github.com/postcss/postcss-import

It felt natural to me to immediately use the @import directive right after installing PostCSS and after fighting a bit with it I realised the plugin wasn't installed so it seemed like it would be a sensible default to have.

Adding the official postcss-import plugin enables the usage of PostCSS's @import directive out of the box so when adding a CSS library (like open-props) or when adding custom stylesheets it works seamlessly.

From the PostCSS-import docs:

PostCSS plugin to transform @import rules by inlining content.
This plugin can consume local files, node modules or web_modules. To resolve path of an @import rule, it can look into root directory (by default process.cwd()), web_modules, node_modules or local modules. When importing a module, it will look for index.css or file referenced in package.json in the style or main fields. You can also provide manually multiples paths where to look at.

Note that this is not the CSS native @import which didn't work on a vanilla installation of Rails with Propshaft. I'm actually not sure if it's supposed to, to be honest.