feat: port Data.Set.Pointwise.Interval (#1481)

- feat: port Mathlib.Data.Set.Pointwise.Interval
- Initial file copy from mathport
- Mathbin -> Mathlib; add import to Mathlib.lean

Author: jcommelin

Mathlib.lean

@@ -546,6 +546,7 @@ import Mathlib.Data.Set.Pairwise
 import Mathlib.Data.Set.Pointwise.Basic
 import Mathlib.Data.Set.Pointwise.BigOperators
 import Mathlib.Data.Set.Pointwise.Finite
+import Mathlib.Data.Set.Pointwise.Interval
 import Mathlib.Data.Set.Pointwise.Iterate
 import Mathlib.Data.Set.Pointwise.ListOfFn
 import Mathlib.Data.Set.Pointwise.SMul