feat: expose the precompiled-mode sources to non-CMake builds

Adds pybind11.get_source_dir() / python -m pybind11 --srcdir and a
srcdir variable in pybind11.pc, so build systems such as Meson can
compile src/pybind11_combined.cpp with PYBIND11_PRECOMPILED defined.

Assisted-by: ClaudeCode:claude-fable-5