Function flatMap`Const`

flatMap<T, TOut>(
selector: IndexedSelector<T, Iterable<TOut, any, any>>,
): (src: Iterable) => Iterable
Maps each element to an iterable and yields every item from the flattened results in source order.

Curried version of _flatMap.
Type Parameters
- T
  Element type produced by the source iterable.
- TOut
  Element type emitted by the projected iterables.
Parameters
- selector: IndexedSelector<T, Iterable<TOut, any, any>>
  Projection invoked with the current value and index to produce an iterable of results.
Returns (src: Iterable) => Iterable
A deferred iterable yielding items from each projected iterable sequentially.
Example
```
const values = [..._flatMap([1, 2], (n) => [n, n * 10])];
console.log(values); // [1, 10, 2, 20]
```
or using the curried version:
```
const values = pipeInto(
  ["a", "b"],
  flatMap((value, index) => [`${value}${index}`, `${value.toUpperCase()}${index}`])
);
console.log([...values]); // ["a0", "A0", "b1", "B1"]
```