feat: add treefmt.nix setup

Add a treefmt setup for a single-entry point format-everything system.
To use it, simply run `nix fmt`. This will in term run nixfmt, prettier
and rustfmt.

Signed-off-by: wucke13 <wucke13+github@gmail.com>

treefmt.nix

@@ -0,0 +1,30 @@
+{ pkgs, ... }:
+{
+  # Used to find the project root
+  projectRootFile = "flake.nix";
+  programs.nixfmt.enable = true;
+  programs.prettier = {
+    enable = true;
+    includes = [
+      "*.css"
+      "*.html"
+      "*.js"
+      "*.json"
+      "*.json5"
+      "*.md"
+      "*.mdx"
+      "*.toml"
+      "*.yaml"
+      "*.yml"
+    ];
+    excludes = [
+      "supply-chain/*"
+    ];
+    settings = {
+      plugins = [
+        "${pkgs.nodePackages.prettier-plugin-toml}/lib/node_modules/prettier-plugin-toml/lib/index.js"
+      ];
+    };
+  };
+  programs.rustfmt.enable = true;
+}