create a shell helper file that is bourne-compatible

The others are left there for backward compatibility. Otherwise,
people would load their env.sh and get an error.

The goal here is to avoid a very annoying but when people use scripts to handle their installation. If the script
s shell is different from SHELL (very likely on ubuntu/debian as the default shell is /bin/bash and /bin/sh points to /bin/dash), sourcing env.sh will attempt to load incompatible shell helpers.

We don't provide (yet) functionality that is dependent on specific shell features (as e.g. autocompletion) so just load a generic bourne-compatible shell file. In addition, this file is going to be the entry point for more (if it happens) as we will need to autodetect which shell we are running on each time it gets loaded (vs. in the ruby code)