[Feature] Stylelint Plugin functionality

[tropicalraisel]

Similar to ESLint, but for CSS and SASS: https://www.npmjs.com/package/stylelint

[versedi]

Already achievable via: https://webpack.js.org/plugins/stylelint-webpack-plugin/

const StylelintPlugin = require('stylelint-webpack-plugin');

Encore.addPlugin(new StylelintPlugin());

written from hand but should work out of the box

[tropicalraisel]

I'm aware; this project is just based around providing features through small statements. I figured Stylelint would be a good addition since ESLint doesn't handle styles. Since it's a plugin maybe it should be used as a loader, but I've seen other issues and PRs about updating the enableEslintLoader() to enableEslintPlugin(). Therefore, if that's acceptable adding Stylelint support should be ok.

[tropicalraisel]

Hold off on this until Stylelint supports PostCSS 8.

[Kocal]

Closing for the same exact reasons from #1267 (comment)