feat: support postcss sugarss

[piecyk]

Hi, this PR adds support for handling https://github.com/postcss/sugarss in css plugin, it's Indent-based CSS syntax for PostCSS.

Biggest change here is how the postcss config is resolve.

For handling .sss we need tell postcss to use the sugarss parser but not for css files. This can be achieved via passing a id that will be passed ctx: { file: string } to config. Similar as https://github.com/webpack-contrib/postcss-loader does it.

Current implementation was caching resolved config, this is not true now as we resolved it per id. Changed it to keep cache for each id, but wondering now if that will give any significant performance improvement, or just increase memory 🤔

Added cachedPostcssConfigPath that will keep path to resolved config this can impact performance.

[piecyk]

Ok, changed to only cache postcss config path 😄

[Shinigami92]

nitpick (non-blocking): I would prefer an initial object at the start of the function body and return it at the end of the function
Then in between you can add dynamically properties and extend it

So something like

  const config = {
    plugins: [require('postcss-nested')]
  }

  if (/\.sss$/.test(api.file)) {
    config.parser = 'sugarss'
    config.plugins.unshift(require('postcss-simple-vars'))
  }

  return config

May need to be extended with some type-safety or so

[Shinigami92]

thought (dx): I'm a bit confused by this line
Could we split it into two lines?
I think it means the same as 🤔

cachedPostcssConfigPath = null
return null

[piecyk]

@yyx990803 what do you think this postcss sugarss ?

[yyx990803]

This is obviously very inefficient - I think a better way to do this is have two cached & resolved config values - one for SugarSS and one for normal CSS.

[aleclarson]

We can cache the resolved config with a hash of the ctx object, but we'll need to replace ctx.file with ctx.type (the file extension without the dot).

[aleclarson]

Another option is to provide a cache function like Babel does it:
https://babeljs.io/docs/en/config-files#apicache

[vitalybaev]

@piecyk is it possible to get this merged? Or maybe I'd use parts of your work?

[patak-dev]

@vitalybaev you can fork this PR and keep working on it to address the issues listed above 👍🏼