Add a simple shell.nix to enable QMK toolchain in shell

[evantravers]

Solves: #8

Description

[tzarc]

Not everyone wants to use nix; this file should be removed. Feel free to add instructions in a comment in the shell.nix file, or some sort of associated readme that explains the setup procedure.

[tzarc]

Nor does everybody run around with direnv — if I’d made that assumption we’d already have one with layout python3 so that we don’t have issues with system packages.

[tzarc]

I’d prefer this go into a util/nix directory and instructions provided on how to enable it. Remember people have full control over their userspace repo; they can commit those to their fork after placing things in the right spot.

[budimanjojo]

Currently I have a flake.nix working locally and I don't use the qmk provided by the stable/unstable nixpkgs repo because they get outdated most of the time. So I use buildPythonApplication to build the package manually because it allows more control on the qmk version. The downside is needing to build the program in the environment and updating this file frequently is needed. But if you want to do it this way, I propose using the unstable branch instead of pinning it to 24.05. Because this will get outdated very fast.