[libspng]: New port

[FantasqueX]

• Changes comply with the maintainer guide
• The name of the port matches an existing name for this component on https://repology.org/ if possible, and/or is strongly associated with that component on search engines.
• Optional dependencies are resolved in exactly one way. For example, if the component is built with CMake, all find_package calls are REQUIRED, are satisfied by vcpkg.json's declared dependencies, or disabled with CMAKE_DISABLE_FIND_PACKAGE_Xxx
• The versioning scheme in vcpkg.json matches what upstream says.
• The license declaration in vcpkg.json matches what upstream says.
• The installed as the "copyright" file matches what upstream says.
• The source code of the component installed comes from an authoritative source.
• The generated "usage text" is accurate. See adding-usage for context.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is in the new port's versions file.
• Only one version is added to each modified port's versions file.

[Cheney-W]

Usage test failed in x64-windows and x64-windows-static triplet:
1.

CMake Error at vcpkg/installed/x64-windows/share/spng/SPNGConfig.cmake:27 (find_dependency):
  Unknown CMake command "find_dependency".

This issue could be fixed by adding include (CMakeFindDependencyMacro) on the previous line of find_dependency(ZLIB REQUIRED) in https://github.com/randy408/libspng/blob/v0.7.4/cmake/Config.cmake.in

CMake Error at F:\VSProject\main\CMakeLists.txt:28 (target_link_libraries):
  Target "main" links to:
    spng::spng
  but the target was not found.

The issue is that under the x64-windows-static triplet, the generated target name is spng::spng_static instead of spng::spng.
For fixing this issue, please modify the content of usage file as below:

From: target_link_libraries(main PRIVATE spng::spng)
To: target_link_libraries(main PRIVATE $<IF:$<TARGET_EXISTS:spng::spng>,spng::spng,spng::spng_static>) 

[FantasqueX]

Thanks for your review. I've fixed those issues and opened a PR to upstream randy408/libspng#263

Tested usage on x64-windows