Add :compressed to stream_modes spec (#10490)

:compressed is already supported here so that gzipped files
can be streamed `File.stream!("log.gz", [:compressed])`.

Author: dominicletz

lib/elixir/lib/file.ex

@@ -111,6 +111,7 @@ defmodule File do
   @type stream_mode ::
           encoding_mode()
           | :append
+          | :compressed
           | :trim_bom
           | {:read_ahead, pos_integer | false}
           | {:delayed_write, non_neg_integer, non_neg_integer}