declare & apply via Nix

[nathanshelly]

TODO

• [ ] test
  • [ ] on new machine
  • [ ] upgrading from previous config
• [ ] update root README.md - at the very least "getting started" section

related issues that can be cleaned up

• [ ] https://github.com/nathanshelly/.files/issues/62
• [x] https://github.com/nathanshelly/.files/issues/101
• [x] closes https://github.com/nathanshelly/.files/issues/104

[nathanshelly]

closing in favor of #111. will incorporate feedback here in that PR