Feature/prepare cxx modules support

[ClausKlein]

How to use it:

bash-5.3$ gmake release
ln -s cmake/CMakeUserPresets.json CMakeUserPresets.json
cmake --preset release --fresh --log-level=TRACE
Preset CMake variables:

  CMAKE_BUILD_TYPE="RelWithDebInfo"
  CMAKE_CXX_EXTENSIONS:BOOL="TRUE"
  CMAKE_CXX_SCAN_FOR_MODULES:BOOL="TRUE"
  CMAKE_CXX_STANDARD="23"
  CMAKE_CXX_STANDARD_REQUIRED:BOOL="TRUE"
  CMAKE_CXX_STDLIB_MODULES_JSON="/usr/local/Cellar/gcc/15.2.0/lib/gcc/current/libstdc++.modules.json"
  CMAKE_EXPORT_COMPILE_COMMANDS:BOOL="TRUE"
  CMAKE_INSTALL_PREFIX:PATH="/Users/clausklein/Workspace/cpp/beman-project/execution26/stagedir"
  CMAKE_PREFIX_PATH:STRING="/Users/clausklein/Workspace/cpp/beman-project/execution26/stagedir"
  CMAKE_SKIP_TEST_ALL_DEPENDENCY:BOOL="FALSE"

-- CMAKE_CXX_STDLIB_MODULES_JSON=/usr/local/Cellar/gcc/15.2.0/lib/gcc/current/libstdc++.modules.json
-- The CXX compiler identification is GNU 15.2.0
-- Checking whether CXX compiler has -isysroot
-- Checking whether CXX compiler has -isysroot - yes
-- Checking whether CXX compiler supports OSX deployment target flag
-- Checking whether CXX compiler supports OSX deployment target flag - yes
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/local/bin/g++-15 - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /Users/clausklein/.direnv/python-3.14/lib/python3.14/site-packages/cmake/data/share/cmake-4.2/Modules/Compiler/CMakeCommonCompilerMacros.cmake:248 (cmake_language):
  CMake's support for `import std;` in C++23 and newer is experimental.  It
  is meant only for experimentation and feedback to CMake developers.
Call Stack (most recent call first):
  /Users/clausklein/.direnv/python-3.14/lib/python3.14/site-packages/cmake/data/share/cmake-4.2/Modules/CMakeDetermineCompilerSupport.cmake:113 (cmake_create_cxx_import_std)
  /Users/clausklein/.direnv/python-3.14/lib/python3.14/site-packages/cmake/data/share/cmake-4.2/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:24 (project)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Detecting CXX compile features - done
-- CMAKE_CXX_COMPILER_IMPORT_STD=23;26
-- CMAKE_CXX_MODULE_STD=ON
-- BEMAN_USE_MODULES=ON
-- CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES=/usr/local/Cellar/gcc/15.2.0/include/c++/15;/usr/local/Cellar/gcc/15.2.0/include/c++/15/x86_64-apple-darwin23;/usr/local/Cellar/gcc/15.2.0/include/c++/15/backward;/usr/local/Cellar/gcc/15.2.0/lib/gcc/current/gcc/x86_64-apple-darwin23/15/include;/usr/local/Cellar/gcc/15.2.0/lib/gcc/current/gcc/x86_64-apple-darwin23/15/include-fixed;/Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/usr/include;/Library/Developer/CommandLineTools/SDKs/MacOSX14.sdk/System/Library/Frameworks
-- Configuring done (1.9s)
-- Generating done (0.3s)
-- Build files have been written to: /Users/clausklein/Workspace/cpp/beman-project/execution26/build/release
ln -fs build/release/compile_commands.json .
cmake --workflow --preset release
Executing workflow step 1 of 4: configure preset "release"

# ...

[coveralls]

coverage: 92.047%. remained the same
when pulling f475ca2 on ClausKlein:feature/prepare-cxx_modules-support
into deecebc on bemanproject:main.

[ClausKlein]

Note: this is still disabled here!