While transforming a selector to a range must occur synchronously,
the individual strategies can be attempted in separate animation
frames.

Also, make this more beautiful.

All of the anchoring types are actually synchronous, for now. Having
promise callbacks between the selector->anchor and anchor->range
conversions sometimes creates race conditions when multiple
annotations anchor concurrently. Punt on this for now.

And hide the cache details from the default implementation that
doesn't use them.

I'm not sure this works because I'm not sure the first then function
will be called synchronously by the render frame invoking resolve.

This makes it more robust against change by storing its description
at the container level rather than a DOM Range object that could get
mutated unpredictably by text node splitting.

Rather than searching PDF pages in order, use the position anchor
(when available) to spiral the search outward from the page that
likely contains the quote.

Rather than searching all the pages in parallel, search them serially
and terminate early with a match.

This is maybe too verbose for now but at least it's explicit and
doesn't assume that anchoring implementations can be fully
responsible for cache invalidation.

I won't try to do anything special here yet, instead just calling
it imperatively. Sort this out later.

This is premature optimization and makes things way more difficult
to reason about.

- anchor / describe both take options
- TextQuoteAnchor takes position as an option but it's not stored
- helper closures sprinkled throughout have fewer _ prefixes

It'll cause more false positives than it helps.

Also change the PDF anchoring to leverage the HTML anchoring by
allowing the options to be passed into the module-level anchor
functions in the pdf and html modules so that the pdf module can
be only responsible for finding the appropriate page and then
delegating.

Avoids falsely anchoring to positions that won't resolve once a PDF
page renders.