Mark std::os::wasi::io::AsFd etc. as stable.

io_safety was stabilized in Rust 1.63, so mark the io_safety exports in
`std::os::wasi::io` as stable.

Fixes rust-lang#103306.

Author: sunfishcode

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

@@ -2,5 +2,5 @@
 
 #![unstable(feature = "wasi_ext", issue = "71213")]
 
-#[unstable(feature = "wasi_ext", issue = "71213")]
+#[stable(feature = "io_safety", since = "1.63.0")]
 pub use crate::os::fd::*;