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

tests/extra_python_package/test_files.py

@@ -33,6 +33,7 @@ UV_ARGS = ["--installer=uv"] if HAS_UV else []
 PKGCONFIG = """\
 prefix=${{pcfiledir}}/../../
 includedir=${{prefix}}/include
+srcdir=${{prefix}}/share/pybind11/src
 
 Name: pybind11
 Description: Seamless operability between C++11 and Python