Fix gulp build

[dunkelziffer]

gulp build didn't work for me. I managed to get it 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. So I created the substeps gulp rp (build roofpig without cleaning /build) and gulp 3x (build 3extras without cleaning /build) and used these new "internal" steps in all the old steps. That made gulp build work again.

• However, I didn't manage to get the tests running. rptest.html builds without errors, but just does nothing when I open it in Firefox. It just shows passes: 0 failures: 0 duration: 0s

fixes #30