Introduce eslint-plugin-prettier to the codebase

Now that the codebase is 'pretty' (https://leipert-projects.gitlab.io/is-gitlab-pretty-yet/prettier/), it could be worth adding eslint-plugin-prettier. This would hand all enforcement of prettier rules over to eslint, which should mean that:

• prettier command-line scripts are no longer required
• the prettier extension is no longer needed in editors
• auto-fix (e.g. on save) only runs once

Overall, this should make setup (and on-boarding) easier for those that want to integrate their editors with both eslint and prettier.

There may be downsides to this approach, please add your thoughts in the comments.