✅ add `treefmt` as module, devShell, check, ci integration

[a-kenji]

Adds treefmt through the treefmt-nix module.

This allows the following integration:

• nix fmt: format the code through nix fmt
• treefmt: exposes a wrapped, configured treefmt inside the devShell
• nix flake check: check and lint the code locally, or the ci

All of these options use the same configuration and will share evaluation cache.

Initalize with the following formatters/linters:

• deadnix
• nixfmt
• rustfmt
• statix
• taplo
• yamllint