upgrade gulp to 4.0.0

[alex-gon]

The changes in this branch are to upgrade gulp to version 4.0.0.

We decided to upgrade gulp because gulp tasks would fail if you were running node v10+.
This issue is explained here: https://github.com/mozilla/pdf.js/issues/10355

[shaymdev]

Was this change made manually or did you merge in a commit that had been made on mozilla/pdfjs/master? They made the upgrade to gulp 4 on their master branch and it's in the latest beta release (which has been in beta for 4 months). I'm wondering if, rather than make similar changes on our branch that could make future merges from master more difficult, we should get this gulp upgrade from master. I can guarantee that their changes on their branch is much more thoroughly tested than ours (not to mention made by more experienced maintainers of this library). We could take their changes now by putting ourselves on the beta release rather than the stable. Or we could live with the workaround of needing an older version of node to build the library until their changes make it to a stable release.

Thoughts @alex-gon?

[alex-gon]

I made these changes manually based on this merge to mozilla master: mozilla#10266
I decided to do it manually because we have a few of our own gulp tasks that also need to be updated.
I am okay with either of your other options.

[shaymdev]

I say let's hold off on the change, I'll find a way to document our situation where newer node won't jive well with the build until we're up to a version that has the gulp 4 upgrade.