Add Visible page numbering

[gautierchomel]

ONIX 175 E205 Visible page numbering informs that e-publication contains visible page numbers.

There is currently no equivalent in schema.org ?

Originally posted by @chrisONIX in w3c/publ-a11y#312 (comment)

[mattgarrish]

This could be a case for a descriptor for pageBreakMarkers, but ideally this shouldn't matter. Whether the author chose visible or hidden text, the hope in providing both options was that reading systems would allow users to toggle between states. (I know this hasn't happened, but I wonder if that's a statement on how much people even care about the choice.)

"Visible" may also not be the best descriptor, as reading systems may make the page number visible in the UI even if it's not in the rendered text of the publication.