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_6.txt

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