Add overflow-x and overflow-y to utilities

[emilybrick]

This is a follow up for this PR . .overflow-auto was put on the team nav to solve for scrolling the navigation on smaller screens. However, doing so resulted in an unnecessary vertical scroll bar as well.

Including overflow-x and y utilities in Primer would help solve for instances where an overflow on only one (x or y) axis is required.

In the PR, as a workaround, we've included custom styles to account for overflow-x: auto and overflow-y: hidden. Once we add these utilities to the library we should go back and clean up the above PR (e.g. remove the custom styling).

[shawnbot]

Another note: we didn't include auto in our first pass at per-axis utilities. Let's add those and wrap this up.

[shawnbot]

We do, in fact, have overflow: auto utilities:

css/src/utilities/layout.scss

Lines 43 to 45 in 7b1f954

	.overflow-auto { overflow: auto !important; }
	.overflow-x-auto { overflow-x: auto !important; }
	.overflow-y-auto { overflow-y: auto !important; }