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/frames/openai/frame_12.txt

@@ -0,0 +1,17 @@
+                                      
+                pooope                
+               pnaaeipn               
+               o  ienpp               
+               epiiinnoi              
+              i oiiiieei              
+              ioeeoaoaio              
+              i   ieniin              
+              i   epniio              
+              op ieiipii              
+              i   eionoa              
+              ie  ionooe              
+               p  oi api              
+               e ooiponi              
+               oaeopinia              
+                n oinee               
+