Add support for std::chrono::year_month_day

[razaqq]

Since c++20 there is https://en.cppreference.com/w/cpp/chrono/year_month_day. Currently the only way to format this is to convert it to a time_point or writing a custom formatter. I think it would be quite useful to support this directly.

There exists an std implementation https://en.cppreference.com/w/cpp/chrono/year_month_day/formatter, but none in {fmt}.

There exists a lot of other calendar points like this, which arent supported either

[vitaut]

Thanks for the suggestion. I think we should do it but let's merge this issue into #3758 which already discusses cases like this.

[zivshek]

@vitaut can we reopen this as a separate issue? The implementation is not trivial if we want to support all types of constructors and functionalities.

[vitaut]

We don't need to support all constructors but let's reopen.