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_22.txt

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