Add NixOS flake

[notchapplez]

Description

Added NixOS flake for better support on NixOS

Related Issue(s)

Fixes #

How to test

Install the flake, or add it into the NixOS configuration.

Documentation

Polyfrost/website#36

[notchapplez]

Comment: the only important files that need to be moved are flake.nix and skia-externals.json

[Wyvest]

can you make sure this replicates whatever is added in NixOS/nixpkgs#545530

[notchapplez]

It is practically the same, except this is packaged
as a nix flake, not a nix package that is going to be submitted to nixpkgs.

[saadndm]

Thanks for the PR, but is there a reason for this flake other than the nixpkgs PR not being merged yet?

The nixpkgs PR is more complete (mainly macOS support). Keeping a separate flake would mostly duplicate that work and be extra maintenance in this repo for every freya/skia update. We also shouldn't be recommending this flake as a default as there is no binary cache for it and building OneClient takes a long time for most people.

I do think we should add a devshell that sets up cargo and minecraft dependencies so that it's easier for people on NixOS to develop OneClient/polyfrost mods.