Add Nix (#177)

* add Nix support

* fix formatter output

* mention Nix in README

* fix common import

* fix frontend old version import

* clarified flake pkgs order

* rm old dataDir option

* comment typo

* fix password assertion

* rm old User/Group logic

* rewrite assertion boolean expr

* General flake touchup

- Rewrite `callPackage` exprs to be more readable
- Add pre-commit support for devShell
- Add direnv support

* add simple test

* use correct test func

nix/common.nix

@@ -0,0 +1,9 @@
+{ lib }: {
+  version = "v3.1.2";
+  meta = {
+    description = "A terrible web ui for yt-dlp. Designed to be self-hosted.";
+    homepage = "https://github.com/marcopeocchi/yt-dlp-web-ui";
+    license = lib.licenses.mpl20;
+  };
+}
+