dfr get-month for dataframe

Gets month from date.

Signature

> dfr get-month

Examples

Returns month from a date

> let dt = ('2020-08-04T16:39:18+00:00' | into datetime -z 'UTC');
    let df = ([$dt $dt] | dfr into-df);
    $df | dfr get-month
╭───┬───╮
│ # │ 0 │
├───┼───┤
│ 0 │ 8 │
│ 1 │ 8 │
╰───┴───╯