feat: add context for c++ namespace definition

[defr0std]

Sometimes in complex c++ headers with several namespaces it's a bit hard to orient in which namespace you are currently working. Typically, there is one "public" namespace, and one "internal", and the file looks like this:

namespace public {
  // some code
namespace internal {
  // some code
}
  // some code
}

The proposal is to add the namespace definition as the context line.