The web extension is just a wrapper around lesspass-pure node module. Source file correspond to the zip downloaded on http://github.com/lesspass/lesspass/archive/${sha256}.zip. {sha256} is the git sha256 use when build was done.
To rebuild the web extension, you need node, yarn and md5sum to check the md5 sum of the files
Tested with:
unzip {sha256}.zip -d /tmp
cd /tmp/lesspass-{sha256}
cd packages/lesspass-pure
yarn install
yarn run build
cd ../../
find packages/lesspass-web-extension/extension/dist/ -type f -exec md5sum {} \;
find packages/lesspass-pure/dist -type f -exec md5sum {} \;