Split proxy into multiple header files

[mingxwa]

As more features getting added to the library, managing everything inside a single proxy.h is no longer scalable. It's time to split it.

Changes

• Split proxy.h into 5 different headers in detail:
  • core.h: Full implementation of P3086
  • proxy_creation.h: Full implementation of P3401
  • facade_creation.h: Full implementation of P3584
  • dispatch.h: operator_dispatch, conversion_dispatch, weak_dispatch, etc.
  • skills.h: skills::format, skills::as_view, skills::rtti, etc.
• Renamed the implementation detail namespace from details to detail.
• Renamed instantiated_t (which doesn't really instantiate a class template) into specialization_t.
• Updated CMake and Meson build files to detect proxy header files systematically.

No functional changes