Capture the page number and label for PDF selections using the same
`PageSelector` type that we use for selections in PDF-based VitalSource books.
This has various uses, but one of the immediate ones is that it will allow us to
do some testing of the VS page range behaviors with PDFs, which are more
convenient to test with.

Related to:

 - https://github.com/hypothesis/client/issues/5986.
 - https://github.com/hypothesis/client/issues/3720 (this proposes a more
   PDF-specific selector that would also capture the page region as a rect).