[bug#33431] [PATCH] gnu: Add yarn

[Jelle Licht]

Hey Andrea,

The yarn tarball in your package is not the actual source code of
yarn; it seems to be an aggregation of multiple javascript files if I
am not mistaken, generated by other tools.

A cursory glance at both package.json and yarn.lock indicates that to
build yarn from source, we need access to at least some of gulp,
webpack and babel. I assume these tools are non-trivial to package, so
perhaps someone else has a workable solution for now.

- Jelle