feat: useYAt

[hannojg]

Why

We might want to render any arbitrary view on the y-axis like the horizontal line. I thus created a useYAt hook so consumers can use that to build anything with it.

I extracted the logic out of the HorizontalLine component, which now uses this hook.

Test plan

I made sure that the horizontal line is still rendering as expected with that change.

[arancauchi]

Thanks so much for this pr :)

I believe there's a conflict preventing it from being merged.