Add shell.nix for easy building anywhere Nix is available

[Krutonium]

Description

This PR adds a shell.nix file that can be used by typing nix-shell on any system that has Nix installed, including MacOS and Linux (Though I can't test it on MacOS.) It pulls in all the dependencies (except the rom) and drops you into a shell where you can simply type make and it'll produce a binary.

Will this Pull Request break anything?

No

Suggested Testing Steps

Install Nix on your system, type nix-shell and then make -j{core count}

[D-Vichnyy]

Is this different from #207 ?