Debug option to write surface and contents flags to output VMF

[SirYodaJedi]

These can be represented as flags, and contents, respectively.
https://github.com/ValveSoftware/source-sdk-2013/blob/2d3a6efb50bba856a44e73d4f0098ed4a726699c/src/utils/vbsp/map.cpp#L2893-L2901

Hammer 4 strips them, but Hammer++ keeps them.

[rihi]

If you are simply trying to access this data for debugging, you can already do so by by enabling Debug mode in the Other GUI panel. This writes the flags as texinfo_flags to each side and the contents to brush_contents to each solid.
Note that using the debug mode generates a lot of debug data which could hinder recompiling the map. It is really only intended for debugging.