[lunasvg] Add cmake usage

[JonLiu1993]

Related issue microsoft/vcpkg#30196, unasvg does not export anything after installation, Add usage for port lunasvg.
Tested the usage via vcpkg successfully by lunasvg:x64-windows:

lunasvg provides CMake targets:

    # this is heuristically generated, and may not be correct
    find_package(unofficial-lunasvg CONFIG REQUIRED)
    target_link_libraries(main PRIVATE unofficial::lunasvg::lunasvg)

[adamkewley]

I'm encountering a similar problem when building lunasvg via ExternalProject_Add for my project. Usage point here:

• https://github.com/ComputationalBiomechanicsLab/opensim-creator/blob/0.5.1/third_party/CMakeLists.txt#L59

The way I am getting around it is to use a custom fork (this) with the changeset described in #138 applied to it.

The changeset here is probably more comprehensive than #138, because it includes describing the installation layout (whereas I'm assuming that the cmake default is going to be ok for downstream consumers)

👍 for writing this up - it's good to see alternate solutions to similar problems