feat(data/finset/basic) eq_of_mem_singleton (#10414)
The `finset` equivalent of [set.eq_of_mem_singleton](https://leanprover-community.github.io/mathlib_docs/find/set.eq_of_mem_singleton/src)
Co-authored-by: Iván Sadofschi Costa <isadofschi@users.noreply.github.com>
