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_23.txt

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