Add common marketing utilities to primer/marketing

[tobiasahlin]

This PR adds a few common utilities to primer/marketing:

• The negative margin spacing scale is extended to also generate scale steps 7-12 for top and bottom margins (mt-n7, mb-8, etc), rather than stopping at 6
• hover-grow-mktg animation utility class, as well as an easing variable ($ease-mktg)
• margin auto class (m-auto), for centering elements with position: absolute, and mt-auto for sticking things to the bottom for flex columns within a flex row or a fixed height element
• z-index utility classes (z-1, z-2, z-3, z-n1, z-n2)
• width-auto and height-auto, for resetting width/height of an image, after its width and height have been set in HTML
• object-fit-cover for making an object cover its container https://developer.mozilla.org/en-US/docs/Web/CSS/object-fit

/cc @primer/ds-core

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[simurai]

We're just about to release 16.0.0. Maybe tomorrow. So let's schedule this for 16.1.0.

Changes look good. Some of these might be handy as non-marketing utilities.. like the z-index and auto margin. I think we wanted to add them for a while already.

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🦋 Changeset detected

Latest commit: 5d2f0be

The changes in this PR will be included in the next version bump.

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@primer/css	Minor

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[tobiasahlin]

So let's schedule this for 16.1.0

That sounds great ✨

Some of these might be handy as non-marketing utilities.. like the z-index and auto margin. I think we wanted to add them for a while already.

Happy to move them out from the marketing bundle if that's the case 🙏