Auxiliary: Add support for parsing C++ in a metadesk format.

[Ed94]

Repo: https://github.com/Ed94/metadesk

I'm currently in the process of lifting latest metadesk from the RAD Debugger into a standalone library of a similar format to gencpp's structure and generation options (c11 & cpp17 support). For future purposes I want to add support for parsing C++ from a metadesk structural format instead of lexing straight from text using a separate auxiliary. It would be provided as a separate extended library that would depend on the base gencpp header.

The AST may need modification to natively support metadesk nodes for tokens, or maybe not... to early to tell how much the Tokens of the lexer will be preserved or adapted to suit either raw text processing and processing with md.

The worst case assumption is that the original lexing and parsing code paths for traditional text parsing may not be partially compatible without it being a great kludge.