Use composer to handle the dependency on pdf.js

Problem/Motivation

The current implementation for Drupal 9 requires manual download and installation of the Mozilla pdf.js library old school (i.e. below the /libraries folder). This is no longer the best practice for Drupal modules requiring third party libraries.

The library is available from packagist: https://packagist.org/packages/clean-composer-packages/pdf-js

Proposed resolution

Use composer to handle the dependency on pdf.js

Remaining tasks

1. Add a composer.json to the project.
2. Make the pdf.js library a requirement by means of this composer.json.
3. Fix the code to autoload this library.