Fix for Compilation Error When Using FMT_MODULE (#3597)

* FMT_EXPORT reorganization to fix compile error

- When compiling with FMT_MODULE, an error was being generated due to
  use of FMT_EXPORT on a declaration within FMT_BEGIN_EXPORT and
  FMT_END_EXPORT on `include/core.h`

- On `include/format.h` had to exclude the function vformat_to by ending
  the export block just before it and beginning a new one after it.

include/fmt/core.h

@@ -1661,7 +1661,6 @@ template <typename Context> class basic_format_arg {
   \endrst
  */
 // DEPRECATED!
-FMT_EXPORT
 template <typename Visitor, typename Context>
 FMT_CONSTEXPR FMT_INLINE auto visit_format_arg(
     Visitor&& vis, const basic_format_arg<Context>& arg) -> decltype(vis(0)) {

include/fmt/format.h

@@ -1038,6 +1038,7 @@ namespace detail {
 FMT_API bool write_console(std::FILE* f, string_view text);
 FMT_API void print(std::FILE*, string_view);
 }  // namespace detail
+
 FMT_BEGIN_EXPORT
 
 // Suppress a misleading warning in older versions of clang.
@@ -4319,6 +4320,8 @@ inline auto to_string(const T& value) -> std::string {
   return to_string(format_as(value));
 }
 
+FMT_END_EXPORT
+
 namespace detail {
 
 template <typename Char>
@@ -4390,6 +4393,8 @@ void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
   detail::parse_format_string<false>(fmt, format_handler(out, fmt, args, loc));
 }
 
+FMT_BEGIN_EXPORT
+
 #ifndef FMT_HEADER_ONLY
 extern template FMT_API void vformat_to(buffer<char>&, string_view,
                                         typename vformat_args<>::type,