DataFrame.group_by.GroupBy.head(n: int) should support negative ints

[thomasb-gr]

Description

polars.DataFrame.head(n) supports negative n to fetch all but the last n rows, but DataFrame.group_by.GroupBy.head(n) does not offer the same functionality. I can work around this by calculating the length manually and then using that, but it seems odd that these two functions behave differently depending on what I'm applying them to.