sqlite_orm as a C++ named module

[trueqbit]

sqlite_orm can be imported as a C++ named module now.

• The named module is exported by the the module interface unit file modules/sqlite_orm.ixx [msvc], modules/sqlite_orm.cppm [clang], modules/sqlite_orm.cxx [gcc].
• Currently the sqlite_orm module in turn consumes the STL as a module, too, via import std.compat;. This means that the C++23 standard named modules std and std.compat must be available.
• The CMake scripts install modules/sqlite_orm.* into ${CMAKE_INSTALL_LIBDIR}/cmake/SqliteOrm/src (following Craig Scott's writeup).
• There's a new test target named "modules_test". It builds the sqlite_orm module and simply imports it in a bare .cpp file, in order to test whether those steps work. Otherwise no specific tests are executed.
• C++ named modules require support from build systems. At the time of writing, CMake only provides experimental support for the dependency on the standard library modules [see CMAKE_EXPERIMENTAL_CXX_IMPORT_STD].
  -> Therefore, the test target “modules_test” is only defined when testing with Visual C++.
  -> Therefore, the named module still has to be consumed manually in a CMake script.

To discuss:

• Because C++ named modules don't export macros, I think we need to provide sqlite_orm feature test macros in a traditional dedicated header file. These macros are e.g. used in examples to decide whether an example should be built, which is a public facing part of sqlite_orm [e.g. SQLITE_ORM_WITH_CTE, SQLITE_ORM_OPTIONAL_SUPPORTED].

[kelteseth]

Nice! Please create a PR when this is done at https://arewemodulesyet.org/ :D