format_to_n doesn't format characters with printf-style format strings

[timwoj]

Similar to #778, but with printf-style format strings:

This code snippet:

char tmp[25];
auto res = fmt::format_to_n(tmp, 25, "%d", 12345);
printf("%lu\n", res.size);
printf("%s\n", tmp);

results in this output:

2
%d

It appears to work completely fine with python-style format strings like {}.

[vitaut]

format_to_n uses Python-style format strings.