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/version.rs

@@ -0,0 +1,2 @@
+/// The current Codex CLI version as embedded at compile time.
+pub const CODEX_CLI_VERSION: &str = env!("CARGO_PKG_VERSION");