Add Stylelint Config for catching bugs and ensuring consistent formatting

[coliff]

I think we should add stylelint and stylelint-config-standard to the projects package.json and the relevant config in the root directory. We can then add a lint-css script to lint the CSS in the assets/css directory. I'd be happy to open a PR for it - let me know...

REF: https://stylelint.io/
https://github.com/stylelint/stylelint-config-standard