std::format_to_n() doesn't work with wchar_t and fmt::runtime()

[n0lavar]

Basically title: https://godbolt.org/z/3sTn6MY4j
Uncomment check<wchar_t>() to reproduce it.

[sunmy2019]

simplified repro.

wchar_t buffer[4];
fmt::format_to_n(buffer, 3, fmt::runtime(L"{}"), 12345);

I will open an PR fixing it.

[n0lavar]

It would be cool to move tests to typed tests so you cover all at once. For instance for me it isn't clear if char16_t will also work (I just remembered UE uses it on Linux). But it's just my mumbling.