chore: Move nix files out of root directory (#5861)

nix/flake.nix

@@ -0,0 +1,27 @@
+{
+  description = "Invidious Nix Flake";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+        devPackages = import ./packages.nix { inherit pkgs; };
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          buildInputs = devPackages;
+        };
+      }
+    );
+}