[sdl2-mixer-ext] add new port

[dbarkar]

• 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.

[jimwang118]

The usage and feature has been tested successfully locally.

find_package(SDL2_mixer_ext CONFIG REQUIRED)
target_link_libraries(main PRIVATE $<IF:$<TARGET_EXISTS:SDL2_mixer_ext::SDL2_mixer_ext>,SDL2_mixer_ext::SDL2_mixer_ext,SDL2_mixer_ext::SDL2_mixer_ext_Static>)