treewide: Add treefmt.toml with configuration to format Nix files

treefmt.toml

@@ -0,0 +1,14 @@
+# One CLI to format the code tree - https://github.com/numtide/treefmt
+[formatter.nix]
+command = "alejandra"
+options = ["-q"]
+includes = ["*.nix"]
+excludes = [
+  # Naturally, I don't want to reformat vendored nixpkgs, otherwise no patch will ever
+  # apply.
+  "nixpkgs/*.nix",
+  # Despite the extension, this file is actually jinja2 tempate for nix file.
+  "cookiecutter/haskell/*/default.nix",
+  # Auto-generated by niv(1)
+  "nix/sources.nix",
+]