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_20.txt

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