Apply Nix3 formatting interface

[HariAmoor-professional]

This module exposes treefmt as a Nix application, i.e., runnable with .#apps.<system>.format. However, Nix3 supports a new formatting interface using the syntax nix fmt (although the program is still highly experimental).

Should we aim to support the new interface? Do we need to involve Numtide in this change, considering that this repo is to be retired after upstreaming as per #3?

[srid]

This repo will be archived.