Add danger rule for legacy CSS utils

What does this MR do and why?

The CSS utils task group is in the process of migrating legacy CSS utilities to the Tailwind equivalent. Legacy utils that do not match Tailwind naming conventions are stored in config/helpers/tailwind/css_in_js.js. These utils will be iteratively migrated to the Tailwind equivalent. We are trying to limit introducing new instances of these legacy utils. To help, this MR introduces a new Danger rule that gives a warning if legacy utils are introduced in a MR.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

Checkout this branch and run bin/rake danger_local