[css-scroll-snap-2] Set pseudo snap Targets to their owning elements. (…

…#10376)

* [css-scroll-snap-2] Set pseudo snap Targets to their owning elements.

This specifies the resolution from
#10175 that when the targets
of a snap event are pseudo elements, the snap event should the owning
elements of the pseudo-elements as snap targets.

* Bikeshed fixes, terminology

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Co-authored-by: David Awogbemila <awogbemila@chromium.org>
Co-authored-by: Tab Atkins Jr <jackalmage@gmail.com>