Eric's Nix(OS|-darwin) configuration

TODOs:

Support multiple hosts
- Change flake.nix to have separate hosts
- Patch the apps/* scripts to take an argument for the host
- Build work and personal nix-darwin/macOS hosts
- Build a NixOS host for Framework 13
- Build a NixOS host for WSL
- Build a NixOS host for a Linux VM
Migrate to sops-nix from agenix
Convert Neovim configuration into a Nix-Lazy hybrid
Separate out home-manager and OS configurations
- Add separate home-manager checks