add is_character

is_character.hpp
```
// Distributed under the Boost Software License Version 1.0 https://www.boost.org/LICENSE_1_0.txt
// Copyright Gero Peterhoff

#ifndef BOOST_TYPE_TRAITS_IS_CHARACTER_HPP
#define BOOST_TYPE_TRAITS_IS_CHARACTER_HPP

#include
#include

namespace boost
{
template struct is_character : public false_type{};
template struct is_character : public is_character{};
template struct is_character : public is_character{};
template struct is_character : public is_character{};

#if defined(__cpp_unicode_characters)
template <> struct is_character : public true_type{};
template <> struct is_character : public true_type{};
#endif

#if defined(__cpp_char8_t)
template <> struct is_character : public true_type{};
#endif

template <> struct is_character : public true_type{};
template <> struct is_character : public true_type{};

#if !defined(BOOST_NO_CXX17_INLINE_VARIABLES)
template inline constexpr bool is_character_v = is_character::value;
#endif
} // boost

#endif // BOOST_TYPE_TRAITS_IS_CHARACTER_HPP
```

Start by reviewing the proposed is_character.hpp and the surrounding type_traits header organization. Verify the listed character types and cv-qualified cases against the project's conventions; the work is done when the new trait is integrated consistently and its supported cases are covered by the repository's existing checks.

Written by the indexing model from the issue text.