ENH: ListDtype / ListArray
This issue is for adding a ListDtype. This might be useful on it's own, and will be useful for https://github.com/pandas-dev/pandas/issues/35169 when we have string operations that return a List of values per scalar element.

I think the primary points to discuss are around

1. How the `value_type` of the List, the `T` in `List[T]`, should be specified by the user
2. How, if at all, to switch between the `list_` and `large_list` types.

xref https://github.com/rapidsai/cudf/issues/5610, where cudf is implementing a ListDtype. Let's chime in over there if we have any thoughts.