Building with C++ compiler

[DenisBiryukov91]

Describe the release item

Currently there are issues when building pico with c++ compiler:

1. All the public api are wrapped in extern "C", while none of the internal code is - which can lead to ub.
2. We are using compound initializers inside internal code - which are not part of C++ standard (even designated initializers are only supported since C++ 20)

It would be good to address this issues.
The first one can be probably fixed by setting a config variable Z_USING_CPP_COMPILER (or something similar) based on the platform/ compiler info from cmake.
To solve the second point, we would need to correct all uses of compound initializers (we can leave designated initializers if we state that we require at least C++ 20 compliant compiler).