feat: introduce a mergeStatus column when joining frames

[teto]

to mimic pandas mergeStatus column: specifies how the merge was done, leftempty, rightempty, present in both frames etc

WIP: I would like some feedback on the API n case it would be ok to upstream.

Right now I struggle with issues like Grouping (Record fs) ■ • Expected kind ‘[(GHC.Types.Symbol, *)]’, but ‘fs’ has kind ‘[*]’

[acowley]

Very ambitious! I still do not really understand how the Grouping stuff works, so I don't have any instant solutions.

Is the TODO suggesting that you want the user to supply the name of the status column, or that we need to do it?

My reading is that a type variable like rs is a list of kind [(GHC.Types.Symbol, *)], so we should append '("merge_status", MergeStatusField) to the list of post-join fields.