[Feature Request] Allow fr units for CSS grid

Lint is failing because it won't allow fr units. However, they are extremely powerful when it comes to spacing elements using the available space. More on it here...

https://alligator.io/css/css-grid-layout-fr-unit/

Here's the error log: https://gitlab.com/gitlab-com/www-gitlab-com/-/jobs/67164153

cc @ayufan