Page margin boxes as an aspect of page setup

[foolip]

https://github.com/web-platform-dx/web-features/blob/main/features/page-setup.yml is a mix of the widely supported @page { size: ... } and the much newer nested at-rules for page margin boxes only support since Chrome 131.

When we have aspects, I think we should make page margin boxes an aspect or its own feature destined to be merged into page setup, depending on what "tools" are added.

[foolip]

If this change is made a change in WPT is also needed, to adjust the mappings made in web-platform-tests/wpt#56694 for page-setup.