Missing `IsTerminal::is_terminal` for files and stdin

As far as I see (correct me if I'm wrong) I don't see any `IsTerminal::is_terminal` alternative known from std lib.

None of [`async_std::fs::File`](https://docs.rs/async-std/latest/async_std/fs/struct.File.html) nor [`async_std::io::Stdin`](https://docs.rs/async-std/latest/async_std/io/struct.Stdin.html) doesn't have any `IsTerminal` implementation.

Are there plans to change it soon?