feat: add gl-translate-y-n50 utility class

What does this MR do?

Adds a gl-translate-y-n50 utility class that is equal to transform: translateY(-50%);. This is helpful when vertically aligning absolutely positioned elements. Specifically this is needed for vertically aligning a lock icon with a label in gitlab!57878 (diffs) and gitlab#324097 (closed)

Does this MR meet the acceptance criteria?

Conformity

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Security

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Accessibility

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