`neovim`: Create standalone config

[Lehmanator]

To-Do

• Remove programs.nixvim prefix from all options in nixvim/neovim profiles
• Call top-level nixvim config with makeNixvimConfigWithModules (or whatever the function name is`.
• Import standalone config in home-manager config.
• Import in devShells.*

Questions

• Is it possible to wrap Neovim with Nixvim config, then later apply extra config from devShells, home-manager, or NixOS system?