Add support for display contents

[nikischin]

Prerequisites

• [x] I have searched for duplicate or closed feature requests
• [x] I have read the contributing guidelines

Proposal

It would be cool for the utility classes to support the display: contents

W3C Mozilla

Motivation and context

I would expect to be able to use a d-contents utility class just like the other utility classes available.

[mdo]

Definitely ripe for the utility API, and this for now isn't super well supported from first glance, so not a thing we can merge into v5.

[mdo]

This has been added in v6. We can do it in v5.4.0 though too.

[nikischin]

This has been added in v6. We can do it in v5.4.0 though too.

Great @mdo! How can I get my PR merged in 5.4.0? :)