feat(nix-compat/nixcpp): init nix.conf config parsing

This allows parsing files like `/etc/nix/nix.conf` into the `NixConfig`
struct.

Change-Id: I90b25f43c429dd56127500ff5068e83852adee13
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11888
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Brian Olsen <me@griff.name>

tvix/nix-compat/src/lib.rs

@@ -3,6 +3,7 @@ pub mod derivation;
 pub mod nar;
 pub mod narinfo;
 pub mod nixbase32;
+pub mod nixcpp;
 pub mod nixhash;
 pub mod path_info;
 pub mod store_path;