src: refactoring and cleanup of node_i18n

Starting some cleanup/refactoring of node_i18n

• Refactor Converter and ConverterObject definitions
  • Move definitions to node_i18n.h
  • Introduce a new utility functions for better encapsulation
  • Adhere closer to c++ style guide
  • Simplify some state tracking
  • Use std::unique_ptr for cleanup

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines