create a portable, use-anywhere dev shell

[jakeisnt]

1. Create a way to manage dotfiles outside of a home-manager configuration; this means creating and building configurations for programs into their packages rather than managing the files external to the packages. this is an elegant approach, but it might be super difficult to refactor with this config (though i don't use home-manager in the best way anyways?)

• https://github.com/bouk/b/blob/master/fish.nix does this, pulling all deps into a custom fish package build

2. expose this through default.nix; can check lib.inNixShell to expose the shell or the configuration depending on the dev environment (though how does this interop with my local nix environments? i don't want to mess up and not rebuild correctly)
3. add a redirect to main branch .tar.gz of this repo with nginx on the isnt.online server if the user has a particular user agent: https://serverfault.com/questions/775463/nginx-redirect-based-on-user-agent. the nix-shell user agent is apparently obvious to detect, but i have to figure out what it uses as the agent string first and make sure it doesn't conflict with normal web browsers.