Support fmt::runtime with wchar_t in fmt::format_to_n

[sunmy2019]

resolves #4714

[vitaut]

Thanks for the PR. We shouldn't modify base.h for this.

[sunmy2019]

Thanks for the PR. We shouldn't modify base.h for this.

How about reimplementing similar functions in xchar.h?

[vitaut]

I think what we need here as an overload of format_to_n that takes wformat_string in xchar.h. Existing runtime format string machinery should take care about the rest as it already does for other overloads.

[n0lavar]

what about char16_t?

[vitaut]

Let's focus on wchar_t in this PR.

[sunmy2019]

Let's focus on wchar_t in this PR.

I use wformat_string in the new implementation. This may remove previous support of char type not in char, wchar. Tell me if it is not desired.