Add Nix flake, devshell, package, and wrapper script

[spikespaz]

I have added a Nix flake to this repository that includes a package description, and a development shell, as well as some of the standard tricks for Nix users.

I have also added a wrapper (cli.nix) so that if a user wants this installed on their system, they can install the package factorio-sat-cli and get one command, factorio-sat, instead of many scripts on their PATH.

There is a significant change to the Python codebase, it fetches assets from $XDG_DATA_HOME. I made those changes a while ago now; if you want me to rebase to make a change, I can.

• nix run github:spikespaz-contrib/factorio-sat will run the wrapper, and show you all of the commands. Then, pass arguments after --, such as nix run github:spikespaz-contrib/factorio-sat -- belt_balancer, which will show the help for the belt_balancer script.
• nix build github:spikespaz-contrib/factorio-sat will fetch dependencies and install the scripts to ./result/bin. This is not the CLI wrapper.
• nix build 'github:spikespaz-contrib/factorio-sat#factorio-sat-cli' will build the CLI wrapper, which lives at ./result/bin/factorio-sat. There are no other scripts or dependencies in bin, the CLI script is self-contained.
• nix develop github:spikespaz-contrib/factorio-sat will put you in a shell with all dependencies and the interpreter. This shell is dual-purpose: it provides the development environment yes, but users can also run this command to get the scripts temporarily on their PATH.
• nix develop (in locally cloned repository) will open the development shell, same as before. Again, this can be used to work on Factorio-SAT itself, or to just run the scripts from PATH.