Convert annotator/anchoring/pdf.coffee to JS

This is a manual conversion to JS.

There is one intended API change which is that `describe` returns a
Promise in all cases, including when a multi-page selection has been
made.

src/shared/polyfills.js

@@ -5,6 +5,7 @@ require('core-js/es6/promise');
 require('core-js/es6/map');
 require('core-js/es6/set');
 require('core-js/es6/symbol');
+require('core-js/fn/array/fill');
 require('core-js/fn/array/find');
 require('core-js/fn/array/find-index');
 require('core-js/fn/array/from');