Convert the code in `src/core/worker.js` to use ES6 classes
Convert the code in `src/core/worker.js` to use ES6 classes

This removes one instance of `// eslint-disable-next-line no-shadow`, which our old pseudo-classes necessitated.

*Please note:* I'm purposely not doing any `var` to `let`/`const` conversion here, since it's generally better to (if possible) do that automatically on e.g. a directory basis instead.

nawawi/pdf.js,showpad/mozilla-pdf.js,xavier114fch/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,caremerge/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,macroplant/pdf.js,nawawi/pdf.js,macroplant/pdf.js,Snuffleupagus/pdf.js,timvandermeij/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,Shoobx/pdf.js,caremerge/pdf.js,mozilla/pdf.js,nawawi/pdf.js,Shoobx/pdf.js,timvandermeij/pdf.js,showpad/mozilla-pdf.js,macroplant/pdf.js,Shoobx/pdf.js,xavier114fch/pdf.js