Add support for c++20 modules and header units

Description:

ttBld needs to add the ability to create modules and header units. Ideally, these would be part of the ninja build script -- though that means we're going to need a way for the user to specify the dependencies. For example, the user might #include "myheader.h" in main.cpp but needs to have /module:reference myheader.h:myheader.h.ifc added to the compile line for main.cpp only, and have a dependency setup between main.cpp and myheader.h.ifc so that ninja can first check whether myheader.h.ifc needs to be rebuilt. All of this would be dependent on a compiler switch -- particularly right now while clang-cl doesn't support these options.

I'm thinking we should support modules first -- I think long term that's going to be the most used. It's also something we could use for the wxUiEditor repository and could affect the code that tool generates.