Fix and refactor gulp

[dunkelziffer]

gulp build and gulp test didn't work for me. I managed to get them running again.

Notes:

• I updated gulp to v4 and created an .nvmrc with Node v16
• Then gulp complained and I had to adjust the syntax slightly to v4
• Then gulp build failed, because it called two other build steps that both cleaned the build directory and deleted each others files. Something similar happened for gulp test. I fixed both issues.

fixes #30