Add support for C++ modules

[mikomikotaishi]

This pull request adds support for C++ modules, enabled in CMake with PLOTLYPP_BUILD_MODULES. It pulls in all headers of the library, including <plotlypp/traces/*.hpp>.

[jimmyorourke]

Thanks!

[jimmyorourke]

Instead of setting include dirs again, let's link against the plotlypp target.
target_link_libraries(plotlypp_module PUBLIC plotlypp)

(I'm wondering if we might need the plotlypp target added to the install export set back in the main cmakelists)

[jimmyorourke]

I have now added the install directives, export set, etc for the plotlypp target to the main cmakelists if you rebase

[jimmyorourke]

Actually I've changed my mind and I think we can make this all much simpler.
I don't think we need a separate CMakeLists, or separate modules target.
In the main CMakeLists, we can bump the min version to 3.28. Then we could do

if(PLOTLYPP_BUILD_MODULES)
    # modules BMI target
    add_library(plotlypp)
    target_sources(...
else()
    # Header only
    add_library(plotlypp INTERFACE)
endif()

We set the cxx_standard conditionally as well, change the linking to public from interface, add the modules export section to the install target (that I just added), and hopefully it should just work...

[mikomikotaishi]

OK, I've applied this change now.

[jimmyorourke]

For now this is fine, but you've got me thinking about whether we should auto generate this file with the rest of the source generation, or create and apply an export macro at source generation time.

[mikomikotaishi]

I think there are some known ways of doing this, such as MSVC's EXPORT_API macro which expands to export when #included into a module.

[jimmyorourke]

I don't have proper unit tests yet for this project, part due to its nature and part due to its stage, (JSON comparisons are probably the best bet), but it might be good to throw together a quick example target that we can ensure compiles. The CI builds have a c++23 variant in which we could enable the modules option.