Provide a pkg-config file (libcxxwrap_julia.pc) during install

[LukasYoder]

At present, libcxxwrap-julia installs CMake package configuration targets (JlCxx::cxxwrap_julia) but does not generate a pkg-config file. Many downstream build systems that are not CMake‐based (Autotools, Meson, plain Makefiles, vcpkg, Spack) discover libraries through pkg-config. The absence of a .pc file forces them to write fragile, hard-coded fall-backs.

[barche]

I won't add or maintain such a file myself, but I would not oppose a PR adding it.