Add example using modules

[jimmyorourke]

Following up from #1, add an example using modules, export the forgotten Layout class, and reduce repetition in the CMake.

To follow:

• Get the C++23 CI using modules
• Autogenerate the exports to avoid forgotten symbols

Also note, apple clang on mac does not support modules, LLVM clang is required: https://stackoverflow.com/questions/79143467/using-c-20-modules-with-cmake-using-appleclang-16-with-macos
CXX=/usr/local/opt/llvm/bin/clang++ cmake -G Ninja .. -DPLOTLYPP_BUILD_MODULES=ON or similar