Add support for additonal packages

Problem/Motivation

We want to make it easier to do Javascript builds. Right now that's not really a problem, because we have nodejs installed on the host itself -- but it's desirable to make sure everyone has the same version of the tools wherever they go.

So it should be possible to specify additional Nix packages to load in the dev shell without having to modify the flake. (It's pretty easy to modify the flake, but then as we add new functionality you need to maintain your changes).

Proposed resolution

Support specifying additional Nix packages either in the .env file, in an optional include file of some type, or both.