Make the whole std::os::wasi::io module stable.

Author: sunfishcode

library/std/src/os/wasi/io/mod.rs

@@ -1,6 +1,6 @@
 //! WASI-specific extensions to general I/O primitives.
 
-#![unstable(feature = "wasi_ext", issue = "71213")]
+#![stable(feature = "io_safety", since = "1.63.0")]
 
 #[stable(feature = "io_safety", since = "1.63.0")]
 pub use crate::os::fd::*;