Changes in text rendering across PDF.js versions can render position
selectors invalid. Therefore any anchoring done with position selectors must
be checked against the quote, as we do with HTML annotations.

This commit disallows anchoring using only position selectors in PDFs
and restructures `anchor` control flow using async/await to make it
easier to follow.

We have been capturing quote selectors with PDF annotations forever, so
there should be no impact on old annotations.