Eliminate shadow variable warning

gcc-9 gives the following shadow warning:
```
In file included from /Users/libraries/ioss/src/fmt/ostream.h:12,
                 from /Users/libraries/ioss/src/Ioss_DatabaseIO.C:59:
/Users/libraries/ioss/src/fmt/format.h: In function 'void fmt::v6::internal::parse_format_string(fmt::v6::basic_string_view<Char>, Handler&&)':
/Users/libraries/ioss/src/fmt/format.h:2442:10: warning: declaration of 'struct fmt::v6::internal::parse_format_string(fmt::v6::basic_string_view<Char>, Handler&&)::writer' shadows a global declaration [-Wshadow]
 2442 |   struct writer {
      |          ^~~~~~
/Users/libraries/ioss/src/fmt/format.h:1703:7: note: shadowed declaration is here
 1703 | using writer = basic_writer<buffer_range<char>>;
      |       ^~~~~~
```

Since the `writer` struct is only used internally in the `parse_format_string` function, its name can be changed somewhat aribtrarily to avoid conflicts with names in an outer scope.

Note that this warning is also present in the 6.0.0 release.

include/fmt/format.h

@@ -2439,7 +2439,7 @@ template <typename Handler, typename Char> struct id_adapter {
 template <bool IS_CONSTEXPR, typename Char, typename Handler>
 FMT_CONSTEXPR void parse_format_string(basic_string_view<Char> format_str,
                                        Handler&& handler) {
-  struct writer {
+  struct pfs_writer {
     FMT_CONSTEXPR void operator()(const Char* begin, const Char* end) {
       if (begin == end) return;
       for (;;) {