Add cookbook chapter on how to read env vars from foreign shell scripts

[remmycat]

This originated from a conversation in the Nushell Discord, where I showed a less sophisticated version of this script in the #cool-scripts channel, and was asked to contribute it to the Cookbook.

Next to reviewing the code, please give your honest opinions on the text, and if you want me to change anything, I appreciate the feedback 🙂

You may of course also just edit anything yourself if that is preferred.

I hope this is helpful to people!

[fdncred]

oooh, nice work!

[kubouch]

Perfect, thanks!