Consider moving time methods and enums out of OS

[aaronfranke]

Issue description: https://docs.godotengine.org/en/latest/classes/class_os.html

The Weekday enum, the Month enum, and methods using them, such as get_date* and get_unix_time_from_datetime, aren't really related to operating systems, so it would make sense if they were moved elsewhere. Perhaps there should be a dedicated class for working with time?

4.0 milestone since this would break compat and therefore we should decide on it before 4.0 is out.

[Anutrix]

When doing so, I recommend replacing gmtime and localtime with thread-safe versions. gmtime_r for Unix and gmtime_s for Windows.