REPL: Refactor output (#16927)

Shuffle `outputs.rs` into individual `outputs/*.rs` files and start
documenting them more.

Release Notes:

- N/A

crates/repl/src/repl.rs

@@ -6,7 +6,6 @@ mod repl_editor;
 mod repl_sessions_ui;
 mod repl_store;
 mod session;
-mod stdio;
 
 use std::{sync::Arc, time::Duration};