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_24.txt

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