Automatically load node modules in devshell

[robbevp]

This PR makes sure that loading the devshell (using direnv or nix develop) automatically includes the node modules.

[chvp]

How does one add a dependency or manually change its version with this in place?

[robbevp]

Ugh, forgot about that.
We could create a script (like we have for the api) to update both the yarn.lock and yarn.nix file.
I would need to be something like

yarn install --modules-location $PRJ_DATA_DIR/tmp_yarn
yarn2nix > yarn.nix
rm -rf $PRJ_DATA_DIR/tmp_yarn

We can't install to node_modules, since this would try to write to the nix store, but yarn v1 requires us to install the modules in order to update the lockfile (this isn't the case for the most recent version of npm or yarn).

There might also be a way to make this work by appending/prepending the NODE_PATH env var, but I haven't been able to try that.

[robbevp]

Just realised that this would also break usage of yarn link, since that would also need to write to the node_modules folder. I guess the benefit of not having to run yarn install is not worth these downsides?

[chvp]

Ah yeah, that's also a big downside. I would say it's not worth it indeed.