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/dots/frame_33.txt

@@ -0,0 +1,17 @@
+                                      
+               ○●●○●○●○·◉○            
+            ○●●··○●○·◉·◉·●·●◉         
+          ○◉○·●●●●○ ·   ●◉··● ●       
+         ●○◉·◉○·●○        ●··○○●      
+        ○◉·◉···◉○○○         ·●○○○     
+        ◉◉●   ○○○○·○●        ◉○◉○○    
+        ··●    ○●●·○·○       ·●··     
+        ●·●●   ●●●○◉○●       ○ ·●     
+        ··· · ●· ◉●●●●◉○○●··●◉○·○·    
+        ○◉·● ◉◉●◉···○○○○○··●○○○··◉    
+        ○○○● ·◉·◉ ◉     ●●● ··●◉○·    
+         ○●○●● ●           ◉◉◉◉○◉     
+          ●○ ●●●◉●      ○●·●◉●○●      
+            ○· ●○●·●◉●○ ◉●○●·●        
+               ○○●●○○○○●●◉◉           
+                   ····