feat(topology/continuous_on): add `tendsto_nhds_within_iff` (#17260)
feat(topology/continuous_on): add `tendsto_nhds_within_iff` (#17260)

We already had lemmas for both directions. This PR only packages them in an equivalence.



Co-authored-by: Rémy Degenne <761511ba67cc85bc4a1e03c457fc34b85543d8ca@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib