Add DateTime week of year formats and getter

[wilbertom]

Hey, I have some Python scripts I am rewriting in Nim. They make use of the week of year while formatting datetimes into strings.

Here I'm porting that functionality. I understand if its not desired in Nim though.

Here are some references where I got some of the code from and other people requiring it:

https://forum.nim-lang.org/t/6320
https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_from_a_month_and_day_of_the_month_or_ordinal_date

I debated making YearWeekRange be 1..53 but went with 0..52 to match Python.

Its worth pointing out this is my first time writing Nim and don't know the internals of DateTime so a strong code review is recommended.

[ringabout]

Hello, @wilbertom thanks for making this PR, see also #17223

[wilbertom]

Hey @xflywind , totally missed that, thank you for pointing me to it.

I'm glad someone else is thinking about this.

I think something missing on #17223 is format/parse support but, that can be added easily once that other PR is merged.