feat: nixify the template

[johnrichardrinehart]

Description

I'm on NixOS and so I try my best to keep project-specific build tools out of my PATH. There are other developers like me who would like to start with this template, but would have to either embrace some system side-effects (installing rollup, npm, etc. to the global PATH) or implement something like the solution in this PR.

You should be able to confirm this behavior with

1. a nix package manager installation or by using https://github.com/DavHau/nix-portable
2. nix build .

The build output will be in ./result. In my case this looks like

$ tree -L 2 ./result    
./result
├── build
│   ├── bundle.css
│   ├── bundle.js
│   └── bundle.js.map
├── favicon.png
├── global.css
└── index.html

Note that nix develop (to spin up a development shell) will currently spit out some warnings since I've chosen to use NODE_PATH (instead of ln -s the node_modules directory in /nix/store) and rollup doesn't seem to use this environment variable. I've opened rollup/rollup#4271 to address this. If this repository's maintainers would like to wait until that issue is resolved (and possibly until a related PR is merged, there) then we can keep this open until then.