Use stylelint as opposed to csslint in DrupalCI

Problem/Motivation

Follow-up to #2865971: Use stylelint as opposed to csslint in core → . This adds stylinelint config - let's use it in DrupalCI. It much better for our purposes since core patches can update it and add plugins.

All the commands below take place in DRUPAL_ROOT/core
To install stylelint

yarn install

This will install Drupal 8's node JS dependencies of which stylelint is one.

To run it on all core css files. Apply this issue's patch and do the following command from DRUPAL_ROOT/core

yarn run lint:css

Proposed resolution

Make DrupalCI use yarn to install dependencies and run linting - hopefully it is already doing that for eslint.

Remaining tasks

User interface changes

None

API changes

tdb