Add stylelint-prettier package for css linting

[tonypaulbarker]

Steps to reproduce

In the localgov_base directory, run npm run lint:css

Results in:

Error: Could not find "stylelint-prettier/recommended". Do you need to install the package or use the "configBasedir" option?

Proposted resolution

Add the package with

npm install stylelint-prettier --save-dev

[markconroy]

You need to cd web/core and do an npm install, then we are piggybacking on the core stylelint stuff.

[tonypaulbarker]

@markconroy that didn't work for me but @andybroomfield pointed me to this work localgovdrupal/localgov_project#184 so I will try that next.