fix: libcxx 19 removes std::char_traits<uint32_t>

[eagleoflqj]

FAILED: fcitx5-hangul/src/CMakeFiles/hangul.dir/engine.cpp.o 
/Users/liumeo/github/emsdk/upstream/emscripten/em++ -DFCITX_GETTEXT_DOMAIN=\"fcitx5-hangul\" -DFCITX_INSTALL_LOCALEDIR=\"/usr/share/locale\" -D_GNU_SOURCE -Dhangul_EXPORTS -isystem /Users/liumeo/github/fcitx5-plugins/build/js/usr/include/Fcitx5/Core -isystem /Users/liumeo/github/fcitx5-plugins/build/js/usr/include/Fcitx5/Config -isystem /Users/liumeo/github/fcitx5-plugins/build/js/usr/include/Fcitx5/Utils -isystem /Users/liumeo/github/fcitx5-plugins/build/js/usr/include/hangul-1.0 -Wall -Wextra  -O3 -DNDEBUG -std=c++17 -fPIC -fvisibility=hidden -fvisibility-inlines-hidden -fPIC -fexceptions -MD -MT fcitx5-hangul/src/CMakeFiles/hangul.dir/engine.cpp.o -MF fcitx5-hangul/src/CMakeFiles/hangul.dir/engine.cpp.o.d -o fcitx5-hangul/src/CMakeFiles/hangul.dir/engine.cpp.o -c /Users/liumeo/github/fcitx5-plugins/fcitx5-hangul/src/engine.cpp
In file included from /Users/liumeo/github/fcitx5-plugins/fcitx5-hangul/src/engine.cpp:8:
In file included from /Users/liumeo/github/fcitx5-plugins/fcitx5-hangul/src/engine.h:10:
In file included from /Users/liumeo/github/fcitx5-plugins/build/js/usr/include/Fcitx5/Config/fcitx-config/iniparser.h:11:
/Users/liumeo/github/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<unsigned int>'
  820 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
      |                                          ^
/Users/liumeo/github/fcitx5-plugins/fcitx5-hangul/src/engine.cpp:41:19: note: in instantiation of template class 'std::basic_string<unsigned int>' requested here
   41 |     for (auto c : ustr) {
      |                   ^
/Users/liumeo/github/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
   23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
      |                             ^