numbat: add module

Numbat is a scientific calculator with full support for physical units.

modules/programs/numbat.nix

@@ -0,0 +1,50 @@
+{
+  lib,
+  config,
+  pkgs,
+  ...
+}:
+
+let
+  inherit (lib) mkIf;
+  cfg = config.programs.numbat;
+  tomlFormat = pkgs.formats.toml { };
+  configDir =
+    if pkgs.stdenv.hostPlatform.isDarwin then
+      "Library/Application Support/numbat"
+    else
+      "${config.xdg.configHome}/numbat";
+in
+{
+  meta.maintainers = with lib.hm.maintainers; [
+    Aehmlo
+  ];
+
+  options.programs.numbat = {
+    enable = lib.mkEnableOption "Numbat";
+
+    package = lib.mkPackageOption pkgs "numbat" { nullable = true; };
+
+    settings = lib.mkOption {
+      type = tomlFormat.type;
+      default = { };
+      example = {
+        intro-banner = "short";
+        prompt = "> ";
+        exchange-rates.fetching-policy = "on-first-use";
+      };
+      description = ''
+        Options to add to {file}`config.toml`. See
+        <https://numbat.dev/doc/cli-customization.html#configuration> for options.
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    home.packages = mkIf (cfg.package != null) [ cfg.package ];
+
+    home.file."${configDir}/config.toml" = mkIf (cfg.settings != { }) {
+      source = tomlFormat.generate "numbat-config" cfg.settings;
+    };
+  };
+}