Add Iterable.groupBy option which returns List of Lists

[deadsoul44]

Iterable.groupBy returns a map but if it returns List of Lists optionally or with another extension, it will be more useful with nested Listview.

[simc]

You could just call .groupBy().values.toList().

[deadsoul44]

Currently, I am doing that way. But if we could add this option, it would be shorter. If you think it is not necessary, we can close the issue.

[simc]

I think it is not something many users need...

[deadsoul44]

People who have nested Listview builder in widget tree and who get data from some API desperately need this and I guess they are not even aware of groupby function or extension. Other than that, maybe there is no use case.