Specify the type of `window.PDFViewerApplication` in src/types/pdfjs.js
add various missing properties required by `PDFIntegration` as well as
adding checks for potentially null/undefined values that were missing
previously.

 - Add a fallback for older versions of PDF.js where `PDFViewerApplication`
   did not have an `appConfig` property [1]
 - Add a missing check for `getPageView` returning `null`

Fixes https://github.com/hypothesis/product-backlog/issues/1205

[1] The property was introduced in https://github.com/mozilla/pdf.js/commit/b6c74f20561b04573cd00dd924484be7f80a09ce