support custom to_chars_float

[bbbgan]

example:

#include <charconv>  // for to_chars
#include <iostream>  // for std::cout

namespace iguana {

template <typename T>
inline char* to_chars_float(T value, char* buffer) {
  std::cout << "call custom to_chars_float with snprintf\n";
  return buffer + snprintf(buffer, 65, "%g", value);
}

}  // namespace iguana
#include <iguana/json_reader.hpp>
#include <iguana/json_writer.hpp>

struct test_float_t {
  double a;
  float b;
};
REFLECTION(test_float_t, a, b);

void user_defined_tochars_example() {
  test_float_t t{2.011111, 2.54};
  std::string ss;
  iguana::to_json(t, ss);
  std::cout << ss << std::endl;
}
int main(void) {
  user_defined_tochars_example();
  return 0;
}

[qicosmos]

LGTM