New plan for CSS

I would like to remove the dependency on `rollup-plugin-postcss`, it has a [very big dependency tree](https://npm.anvaka.com/#/view/2d/rollup-plugin-postcss) and is not very maintained.

What we need from a CSS solution:

- Import rewriting needs to work
- Don't run postcss or anything by default, so it is as fast as possible
- Allow using sass, scss, stylus, less (user has to install them)
- Minimize maintenance burden
- Postcss needs to work (user has to install it)
- Postcss with CSS modules needs to work - this has to integrate with the JS pipeline
- Need to be able to configure any/all transformers, and add additional ones (maybe sass, scss, stylus, less could be something you have to add 1-5 lines of code to enable)

Nice-to-have:

- Resolving from node_modules
- Caching