Support C++20 modules

[gracicot]

Is your feature request related to a problem? Please describe.

C++20 has added support for modules. A big feature we're looking forward is proper componentization. We declare many symbols in the detail namespace. Modules would allow us to only export symbols part of the public API.

Describe the solution you'd like
Since I also plan to support headers for a long time being, having headers with a KANGARU5_CXX_EXPORT that exands to export would add adequate support without sacrificing componentization or legacy support.

Describe alternatives you've considered
Nothing.

Additional context
Having the syntax import kangaru; would be the best.