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/default/frame_23.txt

@@ -0,0 +1,17 @@
+                                      
+            _,+_=+*++=+__             
+         _=|+|\_,_==,|_|*|+_          
+       ,"+|',;*`    "~:+|\;||,        
+      /;|*;/`          _;;\||;\       
+     //|`/'          ,/,,'*/*\|\      
+    ,\|"/`         _***''/*'|~\|,     
+    `||"|         /:/. _|`  "!|'*     
+    ~/| |         |"|,_\,   | */|     
+    ^"\ |+~;=====;=|_*|_"\_ | |~|     
+    |\\_|"="       /`\ \|_\,~ \_|     
+     /'| | `"""""""   '`/;=|_/^/`     
+      ,||_|.             ,/|\^/`      
+       \ |,'/__       _.";*/+/        
+         \=\+;*+\~==_++"-_+*`         
+           "~!*=\~__+__**`            
+