Changes:
1) `#include <boost/pfr...` is now implicitly does `import boost.pfr` if the modules are supported
2) CI now tests modules on Ubuntu 24.04 with existing runtime tests
3) Renamed module to `boost.pfr`
4) CMakeLists.txt now uses modules for `Boost::pfr` target if modules are supported
5) All the library internals now have unconditional module level linkage. `1)` allows users to mix `#include <boost/pfr...` and `import boost.pfr` in user code without ODR-violations.
Significant differences from https://anarthal.github.io/cppblog/modules3:
* PFR uses a `BOOST_PFR_USE_STD_MODULE` macro for `import std;` / `includes` while building module. This allows to use `boost.pfr` module in C++20 and even without usable `std` module.
