switch from pdfjs to pdf-lib

[mojoaxel]

So I tried to find a fix for #42 and ended up switching from pdfjs to pdf-lib internally.

• The add function is now async! So you need to call it with await. This implements Make add async for the non-browser version as well #66.
• This fixes Merge Compressed PDF gives error #42. This also replaces test: add tests for issue #42 #58.
• This fixes Merge Pdf not merging again with other pdf #64.
• This fixes Merged result PDF file is 3x bigger than expected #31.
• This will result in smaller bundles because pdf-lib is treeshackable and already a little smaller than pdfjs.

This is defiantly a BREAKING CHANGE !
Anybody who is using pdfjs should stick with the v3 version!