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

nix/packages.nix

@@ -0,0 +1,19 @@
+{ pkgs }:
+
+# Development packages, this makes Crystal 1.19 (latest available in nixpkgs)
+# and sqlite available in a `nix-shell` or `nix develop` build environment.
+#
+# We dont need to add other dependencies like `libxml2` or `libyaml` here
+# since they are already available by default in the Crystal Nix package
+# (see the output value of CRYSTAL_LIBRARY_PATH when running `crystal env`
+# Ref: https://github.com/NixOS/nixpkgs/blob/nixos-26.05/pkgs/development/compilers/crystal/default.nix#L231)
+#
+with pkgs;
+[
+  # Invidious dependencies
+  crystal_1_19
+  sqlite
+  shards
+  # Utilities
+  nixd # Nixd LSP to format .nix files
+]