Added prepare hook for building directly from source (GitHub)

[RinkeHoekstra]

A standard npm install from source (especially GitHub) did not build all of the packages in lib, dist and esm.

This adds a prepare hook in package.json that ensures that everything is built properly before the rdflib packages is added to node_modules.

This fixes #559

[angelo-v]

I am not sure if I get what you are trying to achieve. Are you taking about something like npm install git+ssh://...?

I am against using the prepare hook to build things, as it slows down every npm install. If you want to build, then use the build scripts. If you need to install dev snapshots that match the github code, then we should publish them via the pipeline. You can also checkout and build the github code and then install the local module in your project.