pdfjs: upgrade to pdf.js v4

* the new assets are basically vanilla js targeting the browser already
* this doesn't require a build step and allows the assets to be placed
  in the static directory, avoiding a build step and allowing for
  deduplication across themes
* the jinja template is also deduplicated similarly, by being placed
  outside of any theme directory
* implemented functionality with pdfjs-dist v4:
> page navigation: previous/next/jump
> zoom
> fullscreen

Co-authored-by: Carlin MacKenzie <[email protected]>