tex-fmt: add module

tex-fmt is a LaTeX source code formatter written in Rust, and uses
a user configuration file in the .toml format.

modules/programs/tex-fmt.nix

@@ -0,0 +1,53 @@
+{ pkgs, config, lib, ... }:
+
+let
+
+  inherit (lib) mkEnableOption mkPackageOption mkOption literalExpression;
+
+  configDir = if pkgs.stdenv.isDarwin then
+    "Library/Application Support"
+  else
+    config.xdg.configHome;
+
+  tomlFormat = pkgs.formats.toml { };
+  cfg = config.programs.tex-fmt;
+
+in {
+  meta.maintainers = with lib.maintainers; [ mirkolenz wgunderwood ];
+
+  options.programs.tex-fmt = {
+    enable = mkEnableOption "tex-fmt";
+
+    package = mkPackageOption pkgs "tex-fmt" { };
+
+    settings = mkOption {
+      type = tomlFormat.type;
+      default = { };
+      example = literalExpression ''
+        {
+          wrap = true;
+          tabsize = 2;
+          tabchar = "space";
+          lists = [];
+        }
+      '';
+      description = ''
+        Configuration written to
+        {file}`$XDG_CONFIG_HOME/tex-fmt/tex-fmt.toml` on Linux or
+        {file}`$HOME/Library/Application Support/tex-fmt/tex-fmt.toml` on Darwin.
+        See <https://github.com/WGUNDERWOOD/tex-fmt> and
+        <https://github.com/WGUNDERWOOD/tex-fmt/blob/master/tex-fmt.toml>
+        for more information.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    home.packages = [ cfg.package ];
+
+    home.file."${configDir}/tex-fmt/tex-fmt.toml" =
+      lib.mkIf (cfg.settings != { }) {
+        source = tomlFormat.generate "tex-fmt-config" cfg.settings;
+      };
+  };
+}