Add support for alternative month format

[Iamnotagenius]

In ru_RU.UTF-8 locale, there's a possibility to specify alternative month format.
For example, specifying %B format, strftime will output this (providing that said locale is in charge): марта.
It works well in scenarios like %d %B, which will result in 31 марта. However, strftime has support for %OB which will output Март with the russian locale. This will be more appropriate to use in formats like %OB %Y which results in Март 2024.
It seems this functionality is not at all present in this library, when I try to format a date like %OB the library throws an invalid format exception.

[vitaut]

%OB seems non-standard: https://eel.is/c++draft/tab:time.format.spec. Where is it documented?

[jmkuhn]

Some discussions about standardizing this in C:
https://www.austingroupbugs.net/view.php?id=1166
https://www.austingroupbugs.net/view.php?id=258