Add accessibility labels to the diff file line numbers

What does this MR do and why?

This adds aria-labels to the line numbers rendered in a diff file.

Caveats

For now, this simply renders the same number as the attribute with no additional changes.

It may be better for accessibility to include some text like Line [x] so it's announced with more context. However, doing so will dramatically complicate the diff row component and potentially have a noticeable effect on page performance. For now - until further discussion and analysis - just the line numbers should be shipped as-is.

Screenshots or screen recordings

No visual changes.

MR acceptance checklist

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