Refine Enum.chunk_while/4 return type to [chunk] (#15552)

Unlike `chunk_by/2` and `chunk_every/2,4` (which always chunk into
sublists and are typed `[list]`), `chunk_while/4` lets the reducer emit an
arbitrary term as a chunk (`{:cont, chunk, acc}`, with `chunk: any`), so
`[chunk]` is the precise return type.

Author: nseaSeb

lib/elixir/lib/enum.ex

@@ -618,7 +618,7 @@ defmodule Enum do
           acc,
           (element, acc -> {:cont, chunk, acc} | {:cont, acc} | {:halt, acc}),
           (acc -> {:cont, chunk, acc} | {:cont, acc})
-        ) :: Enumerable.t()
+        ) :: [chunk]
         when chunk: any
   def chunk_while(enumerable, acc, chunk_fun, after_fun) do
     {_, {res, acc}} =