API: Rename BaseGrouper.recons_codes to BaseGrouper.reconstructed_codes
Rename ``BaseGrouper.recons_codes`` to ``BaseGrouper.reconstructed_codes``. This is an internal attribute, so ok to rename. 

Also adds some typing.
this needs to coordinate with #28634 (so may need to rebase here once that is merged) The other PR is from a first time contributor that's been at it for a month and a half, so I think let's merge that first and rebase here to keep things moving @topper-123 if you can rebase; might need to type the added method