Infer `pageBreakMarkers`

[HadrienGardeur]

With the work planned on HTML parsing in readium/go-toolkit#180, we should also be in a position to infer the presence of page break markers.

These page break markers can either use:

• the doc-pagebreak role in DPUB ARIA
• or the pagebreak type when using EPUB types

The pageBreakMarkers value for accessibilityFeature doesn't require completeness, which means that the mere presence of one of these markers is sufficient to trigger the inference.