[wasmtime-wasi] make StdinStream and StdoutStream public (#7150) (#7176)

This allows embedders to implement those traits themselves rather than be
restricted to using the built-in implementations.

Fixes #7149

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

crates/wasi/src/preview2/mod.rs

@@ -45,7 +45,9 @@ pub use self::filesystem::{DirPerms, FilePerms, FsError, FsResult};
 pub use self::network::{Network, SocketError, SocketResult};
 pub use self::poll::{subscribe, ClosureFuture, MakeFuture, Pollable, PollableFuture, Subscribe};
 pub use self::random::{thread_rng, Deterministic};
-pub use self::stdio::{stderr, stdin, stdout, IsATTY, Stderr, Stdin, Stdout};
+pub use self::stdio::{
+    stderr, stdin, stdout, IsATTY, Stderr, Stdin, StdinStream, Stdout, StdoutStream,
+};
 pub use self::stream::{
     HostInputStream, HostOutputStream, InputStream, OutputStream, StreamError, StreamResult,
 };