feat: include comments in the cpp generator

[2-towns]

This PR brings the comments from _interface.h into the generated version to provide a better developer experience.

It requires passing the --interface argument explicitly for two reasons:

1. To avoid breaking the default behavior.
2. To explicitly specify the path and avoid relying on implicit imports, which can become messy with CMakeLists and Nix.

Only comments starting with // are supported in header files, as they are easier to handle.