Convert examples/node/pdf2png/pdf2png.js to await/async

[janekotovich]

Because

Pdf2png.js uses chain of promises

This pull request

Convert examples/node/pdf2png/pdf2png.js to await/async

Issue that this pull request solves

Closes: #14126

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

[janekotovich]

@Snuffleupagus Thanks heaps for help and sorry for misunderstanding.
I took into account all advices and hope this time pull req would be good 🤗
I tested patch with:

gulp dist-install 
gulp server
npm install
npm install canvas
cd examples/node/pdf2png
node pdf2png.js

and all seemed to work well. Let me know If you prefer me squashing commits in one 😊
Regards, Jane

[Snuffleupagus]

I tested patch with:
...
and all seemed to work well.

Thanks you!

Let me know If you prefer me squashing commits in one blush

Yes, please squash the commits (we generally use don't separate fixup commits in the PDF.js project).

[janekotovich]

@Snuffleupagus Thanks Jonas all done! ✅

[Snuffleupagus]

r=me; thank you for the patch!