This updates simdutf to my fork which has a SIMDUTF_NO_LIBCXX option that removes all libc++ and libc++ ABI dependencies. From there, the hand-written simd code we have has been updated to also no longer use any libc++ features. Part of this required removing utfcpp since it depended on libc++ (`<iterator>`). libghostty-vt now only depends on libc.