add nix flake

- add following functionality:
  - nix develop with direnv support
  - nix run and nix shell
  - nix fmt for flake.nix
  - and package overlay for https://github.com/NixOS/nixpkgs/pull/225502

- useful docs
  - https://stackoverflow.com/questions/53272197/how-do-i-override-the-libc-in-a-nix-package-to-be-musl
  - dd3aca2d0b/pkgs/top-level/stage.nix (L136)

- inspired by https://github.com/typst/typst/blob/main/flake.nix

README.md

@@ -50,6 +50,13 @@ A pure [Rust](https://www.rust-lang.org/) CLI for [Crunchyroll](https://www.crun
 
 Check out the [releases](https://github.com/crunchy-labs/crunchy-cli/releases) tab and get the binary from the latest (pre-)release.
 
+### ❄️  The nix way
+
+This requires [nix](https://nixos.org) and you'll probably need `--extra-experimental-features "nix-command flakes"` depending on your configurations.
+```shell
+$ nix <run|shell|develop> github:crunchy-labs/crunchy-cli
+```
+
 ### 🛠 Build it yourself
 
 Since we do not support every platform and architecture you may have to build the project yourself.