`polars len` for dataframe

Return the number of rows in the context. This is similar to COUNT(*) in SQL.

Signature

> polars len {flags}

input	output
any	polars_expression

Count the number of rows in the dataframe.

> [[a b]; [1 2] [3 4]] | polars into-df | polars select (polars len) | polars collect
╭───┬─────╮
│ # │ len │
├───┼─────┤
│ 0 │   2 │
╰───┴─────╯

Creates a last expression from a column

> polars col a | polars last