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_8.txt

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