Add missing rounded-3 utility

[esdraspavon]

Closes #1154

/cc @primer/ds-core

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[simurai]

@esdraspavon Thanks for the addition. 🙇

Just realized we also have utility classes for each edge:

css/src/utilities/borders.scss

Lines 55 to 71 in c752b4e

	.rounded#{$variant}-#{$edge}-1 {
	@each $corner in $corners {
	border-#{$corner}-radius: $border-radius / 2 !important;
	}
	}
	.rounded#{$variant}-#{$edge}-2 {
	@each $corner in $corners {
	border-#{$corner}-radius: $border-radius !important;
	}
	}
	.rounded#{$variant}-#{$edge}-3 {
	@each $corner in $corners {
	border-#{$corner}-radius: $border-radius * 2 !important;
	}
	}

Maybe we should use the $border-radius-[1-3] variables too instead of / 2 and * 2 so the values match?

[esdraspavon]

@simurai np! sounds good to me!

[simurai]

✨ Thanks 🙇