Add more date time functions

[skyqrose]

Addresses #1132

And I figured while I was at it I would add the rest of the missing datetime functions from Polars.

I added:

• day_of_month
• is_leap_year
• quarter_of_year (Polars calls this quarter)
• iso_year
• nanosecond

I skipped:

• millisecond and microsecond, which are in most of the Polars datetime traits, but they're missing from https://docs.rs/polars/0.52.0/polars/series/struct.Series.html
• days_in_month, which wasn't added until Polars 0.51 (Explorer is on 0.49. Latest is 0.52)

I also reordered the functions. They're now sorted by:

• date: year, month, day
• Other gregorian calendar functions: is_leap_year, quarter_of_year, day_of_year
• ISO calendar / week functions: iso_year, week_of_year, day_of_week
• times: hour, minute, second, nanosecond

And I also made a couple minor tweaks to tests and fixed a typo or two.

[billylanchantin]

Beautiful work, no notes! I always appreciate attention to detail ❤️