[proposed enhancement] Add optional C++20 module support

[Theramar]

Instead of downloading semver.hpp users could install an optional semver.cppm

simple solution:

//semver.cppm. This is an automatically generated file
#define SEMVER_MODULE
//copy "semver.hpp" content here

//semver.hpp
#ifdef SEMVER_MODULE
#define SEMVER_EXPORT export
#else
#define SEMVER_EXPORT
#endif

SEMVER_EXPORT namespace semver { 
//...
}