Remove sass-lint while switching to stylelint

[vanitabarrett]

What

This PR introduces stylelint alongside our existing linting (using sass-lint). We need to remove sass-lint, so we're only using stylelint.

Why

GOVUK needs to move away from scss-lint as it's blocking their upgrade from Ruby SASS to Sassc. They have chosen stylelint and have raised a PR against GOVUK Frontend to demonstrate how we may switch to stylelint to match. This will also be shared with the frontend community so we can aim for consistency across GDS.

[vanitabarrett]

WIP PR raised, but waiting to see if anything new comes out of #1897

[m-green]

Hi @vanitabarrett - could you let me know when you start work on this so we can take a look at documentation changes in plenty of time?

[vanitabarrett]

@m-green Yep, will do! Waiting for #1897 , and then will ping you once we've made some decisions on that 🙂