Extend the matching between focus filters and document segments to support
matching page range focus filters against VitalSource chapter page ranges.

In the case of VitalSource PDF books, each page is a separate segment, so this
matching is simple. For EPUB-based books a segment can span a range of pages,
and the `<mosaic-book>` element doesn't provide an API rather returns the page
range directly as far as I can see. Instead we can use
`MosaicBookElement.getPageBreaks` to get a list of the CFIs and corresponding
labels of pages, which can then be matched against the current segment's CFI.

 - Add `SegmentInfo.pages` field which contains a page range for a
   segment

 - Support matching `SegmentInfo.pages` against focus filters in
   `segmentMatchesFocusFilters`.

 - Display data from `SegmentInfo.pages` in "Version" tab in sidebar

 - Add `getPageBreaks` API to VitalSource interfaces and use the data
   from this API to populate the `SegmentInfo.pages` field in the VS
   integration.

 - Extend the fake VitalSource books in the dev server to return fake page break
   information.