Add day_of_year and week_of_year

[billylanchantin]

Description

Closes: #714

This PR adds the following date functions:

Explorer	Polars	Description
day_of_year	ordinal_day	Day of year from 1 to 366
week_of_year	week	Week of year from 1 to 53

Notes

Rather than matching Polars exactly, I chose to follow the x_of_y pattern established by day_of_week.

[josevalim]

Beautifully done, thank you @billylanchantin! 💚 💙 💜 💛 ❤️

I just changed the docs to keep the first paragraph a short sentence, as that is used as a summary by the docs. :)

[billylanchantin]

You're welcome!

I just changed the docs to keep the first paragraph a short sentence, as that is used as a summary by the docs. :)

Noted! I'll remember that for the future.