Add support for C++20 modules

Qbs should support C++ modules, which are now available in latest Visual Studio and Clang compilers.

Modules are a much anticipated feature of future C++ that allows to define a clean interface between C++ libraries/plugins/ .... In addition to having a cleaner defined API, it also promises much accelerated compile times. It will probably be part of C++20, but already now MSVC and upcoming Clang support parts of it.

Qbs should support the relevant compiler flags and new file suffixes in a common way, so people can make use of modules early on.