fix(structured-list): top align header text

[emyarod]

Closes #8157
related #721
related #753

This PR top aligns headers so that they stay aligned with non-header text in structured lists

Testing / Reviewing

Apply the header class to a list row cell and check its vertical alignment

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[tay1orjones]

Looks good to me 👍

[tw15egan]

Should we check if the header is part of tbody, and if so change the line-height to match the rest of the row? It's currently getting the header line-height of 1.29 but the rest of the row has a 1.43 line-height

[emyarod]

the line heights are set by our type mixins so we will need to see if they need to be updated

[laurenmrice]

Looks good 🚀

[tw15egan]

LGTM 👍 ✅

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