Add C++20 module build target

Usage in client cmake:
```cmake
set(FLECS_M0DULE ON CACHE BOOL "Build flecs module" FORCE)
add_subdirectory(path/to/flecs)
target_compile_features(flecs-module PUBLIC cxx_std_23)
target_link_libraries(
   someProjectName
   PRIVATE
   flecs-module
)
```

In the code, it is then possible to use `import flecs;` to import all the flecs symbols.
The current version includes everything in the `flecs` namespace, but avoids including anything in the `_` namespace.

Maintainance:
- namespaces that contain symbols to exported must be tagged with FLECS_API_NAMESPACE
- namespaces that should be excluded should be seperate in order to avoid exporting symbols
- static globals must be tagged with `FLECS_STATIC_IN_HEADER`, which becomes `static` in headers, but not in the module
- some rare symbols outside namesapces are necessary for compilation, such as operator new / delete, and must be tagged FLECS_API_DEPENDENCY

CMakeLists.txt

@@ -7,6 +7,7 @@ option(FLECS_STATIC "Build static flecs lib" ON)
 option(FLECS_SHARED "Build shared flecs lib" ON)
 option(FLECS_PIC "Compile static flecs lib with position independent code (PIC)" ON)
 option(FLECS_TESTS "Build flecs tests" OFF)
+option(FLECS_M0DULE "Build flecs C++20 module" OFF)
 
 include(cmake/target_default_compile_warnings.cmake)
 include(cmake/target_default_compile_options.cmake)
@@ -75,6 +76,10 @@ if(FLECS_TESTS)
     add_subdirectory(test)
 endif()
 
+if(FLECS_M0DULE)
+    add_subdirectory(module)
+endif()
+
 message(STATUS "Targets: ${FLECS_TARGETS}")
 
 # define the install steps