[llnl-units] Add new port

[petersteneteg]

Adds the https://github.com/LLNL/units units library.

It's named llnl-units, to avoid conflicts, this has upstream support see LLNL/units#164

• 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.

[MonicaLiu0311]

The usage test passed on x64-windows(header files found):

llnl-units provides CMake targets:

    find_package(llnl-units CONFIG REQUIRED)
    target_link_libraries(main PRIVATE llnl-units::units)

[BillyONeal]

Thanks for the new port!