Type definitions for array.prototype.flatmap (#29437)

* Type definitions for array.prototype.flatmap

* array.prototype.flatmap: importing "/auto" extends the Array prototype

types/array.prototype.flatmap/implementation.d.ts

@@ -0,0 +1,7 @@
+// This is the same type as the callable signature in `FlatMap` in `index.d.ts`.
+declare function flatMap<A, B, T extends object | undefined = undefined>(
+    xs: ReadonlyArray<A>,
+    fn: (this: T, x: A, index: number, array: A[]) => B[],
+    thisArg?: T
+): B[];
+export = flatMap;