Fix nanoplots listcols

[machow]

This PR ensures Great Tables can render a polars DataFrame that has a pl.List column. Previously, it would error, and the error was early enough that formatters couldn't work with this data.

(Note that most formatters are not designed to work with list columns. We may need to support this down the road, or add helpful messaging when erroring in these cases).

(Note also that this might fail for complex list schemas, eg. nested lists like pl.List(pl.List(int)) rather than pl.List(int). But this case also fails in the current code).

Here's a hokey example displaying a table with list columns:

from great_tables import GT
import polars as pl

GT(pl.DataFrame({"x": [[1, 2], [3]], "y": [[.9], [1.1]]}))

And with a tasty nanoplot

GT(pl.DataFrame({"x": [[1, 2], [3]], "y": [[.9], [1.1]]})).fmt_nanoplot("x")

[machow]

Hmm.. I wonder if there's a better way to test this simple case, but it might be a resolve down the road situation 😭

[rich-iannone]

Looks great, thank you!