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/tests/test_backend.rs

@@ -0,0 +1,4 @@
+#[path = "../src/test_backend.rs"]
+mod inner;
+
+pub use inner::VT100Backend;