I m working on this:
bugs.limesurvey.org/view.php?id=14260

my git status says:

modified: assets/packages/adminpanel/build/hammer.min.js
modified: assets/packages/adminpanel/build/lsadminpanel.debug.js
modified: assets/packages/adminpanel/build/lsadminpanel.js
modified: assets/packages/adminpanel/build/lsadminpanel.min.css
modified: assets/packages/adminpanel/build/lsadminpanel.min.js
modified: assets/packages/adminpanel/build/surveysettings.min.js
modified: assets/packages/adminpanel/package-lock.json
modified: assets/packages/adminpanel/src/components/subcomponents/_questionsgroups.vue

I had to install node, npm, gulp etc
to get this working I had to delete the package-lock.json before npm install and run gulp to create new build files.

do I really need to commit all the build files and package-lock.json?

The short answer is "yes".

The files in the build folder are the ones loaded by LimeSurvey in the end.

Since we don't want anyone other than the developers to have to install the whole npm bunch we ship it 'complete'. This is not recommended, I know, but since we want to ship LimeSurvey as a all in one solution with "out of the box" feeling, we have decided to do so.