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

nix/shell.nix

@@ -0,0 +1,11 @@
+{
+  pkgs ? import <nixpkgs> { },
+}:
+
+let
+  devPackages = import ./packages.nix { inherit pkgs; };
+in
+
+pkgs.mkShell {
+  buildInputs = devPackages;
+}