Create shell.nix for quick Nix(OS) setup

[MultisampledNight]

This allows nix or NixOS users to just run nix-shell in the repo and get almost everything listed in the getting started guide up and running automagically. Worth noting that following steps need to be taken manually:

• Git LFS needs to be set up manually, if it's not installed user-wide already.
• The Android SDK license has to be accepted manually. If you run nix-shell the first time in this repo, it'll error, print the license and tell you how to accept it (through nixpkgs configuration).
• Most likely you'll need to set up udev rules for Android. This is also on the system level, though if on NixOS, setting programs.adb.enable = true; and adding your user to the adbusers group suffices, see the Android page in the inofficial wiki.
• For the simulator, there needs to be a working Vulkan setup. This is configured on the system level and also a bit manufacturer-dependent, though if on NixOS, following the advice in the manual (https://nixos.org/manual/nixos/stable/index.html#sec-gpu-accel-vulkan) did the job for me.

Adding --package crab-saber to the cargo apk command was required for me to get it even running, else it complained about not being sure which workspace member to pick.

Note: Since I have no VR device, Android deployment was tested using my mobile phone. I did observe the app building and launching on my phone, however, it gave no display output (but errored still just fine and displayed that on the host PC).

[MultisampledNight]

(I'd add the documentation to the wiki after merge.)

[MultisampledNight]

clippy failure seems unrelated to the changes made.

[kanerogers]

Thanks so much for this @MultisampledNight ! Yeah, don't worry about the clippy stuff, that's clearly unrelated.

I don't have access to a nix environment to test this, but given the small impact of a change like this, I'm happy to merge this in for now and address any issues if they arise.