Refactor Nix packaging

[spitulax]

Changes:

• The Rust toolchain is now obtained from rust-overlay
• Added devShell
• Moved Nix related files into nix/
• Formatted Nix files

[H3rmt]

Can you guarantee that this wont break any existing installs for nix users?

[H3rmt]

Could you load the title, description, etc. from the toml instead of hardcoding them

[spitulax]

Could you load the title, description, etc. from the toml instead of hardcoding them

See NixOS/nixpkgs#277994 on why it's a bad practice.

[spitulax]

Can you guarantee that this wont break any existing installs for nix users?

If a user just use the package, for example inputs.hyprswitch.packages.x86_64-linux.hyprswitch or inputs.hyprswitch.packages.x86_64-linux.default it should be fine. I didn't remove anything from the flake outputs or any of meta attributes from the package. Remind me though if you realise something stopped working.