devel/libxsd-frontend: fix build with libc++ 19

As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.

This causes www/qt5-webengine to fail to compile with clang 19 and
libc++ 19, resulting in errors similar to:

/usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<XSDFrontend::Bits::None>'
  820 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
      |                                          ^
/wrkdirs/usr/ports/devel/libxsd-frontend/work/libxsd-frontend-2.1.0/libxsd-frontend/xml.hxx:100:23: note: in instantiation of template class 'std::basic_string<XSDFrontend::Bits::None>' requested here
  100 |       NarrowString r (s);
      |                       ^
/usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
   23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
      |                             ^

The Bits::None construct used by libxsd-frontend is meant to disable
unwanted conversions, but it no longer works if std::char_traits> is
undefined for this type.

Fix it by removing the Bits namespace and only enabling the converting
constructors if the first type of StringTemplate is wchar_t.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals

PR: 281478
Approved by: maintainer timeout (2 weeks)
MFH: 2024Q3

(cherry picked from commit c1c4aae636e9e8c5fc146caeb9ad734e20f97115)