add `indexedFilterMap`

Implement `indexedFilterMap : (Int -> a -> Maybe b) -> ...` as a more performant and convenient alternative to
```elm
Array.indexedMap Tuple.pair
    |> Array.filterMap (\( i, el ) -> ...)
```
a "findAll" would then be trivial
```elm
Array.filterMapIndexed (\i el -> ...)
```
Related: https://github.com/elm-community/array-extra/issues/12