Move TUI on top of app server (parallel code) (#14717)

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.

Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.

codex-rs/tui_app_server/src/bin/md-events.rs

@@ -0,0 +1,15 @@
+use std::io::Read;
+use std::io::{self};
+
+fn main() {
+    let mut input = String::new();
+    if let Err(err) = io::stdin().read_to_string(&mut input) {
+        eprintln!("failed to read stdin: {err}");
+        std::process::exit(1);
+    }
+
+    let parser = pulldown_cmark::Parser::new(&input);
+    for event in parser {
+        println!("{event:?}");
+    }
+}