Fix C++ module usage

[RobLoach]

Fixes #420

Consuming raylib_cpp_modules failed with "the compiler does not provide a way to discover the import graph dependencies" because C++ module scanning requires a Ninja/Visual Studio generator and a scan-capable compiler, which wasn't checked or documented, and the module path had no test.

• Harden modules/CMakeLists.txt: warn with an actionable message when the generator can't scan for modules, and clean up the target setup.
• Add an import raylib; example built only when modules are enabled.
• Add a CI job that builds the modules with Ninja + Clang so it stays working.
• Document module requirements and a FetchContent consumer template in the README.