feat(data/finset/basic): disj_Union (#16421)
`finset.disj_Union` is to `finset.bUnion` as:
* `finset.disj_union` is to `finset.union`
* `finset.cons` is to `insert`
* `finset.map` is to `finset.image`
Co-authored-by: Yaël Dillies <yael.dillies@gmail.com>
