bump pdfjs-dist to >3

[kpsherva]

    since you are on this, what you if you try to bump `pdfjs-dist` to `>3`? (and check if it works) :)

Originally posted by @ntarocco in #168 (comment)

[max-moser]

Further hints provided by @kpsherva:

Semantics of the # syntax: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields
Webpack issue: webpack/webpack#10216

[carlinmack]

Related mozilla/pdf.js#17953 (comment)