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/shapes/frame_7.txt

@@ -0,0 +1,17 @@
+                                      
+             ◆△◇●□□●●□▲◆              
+          ▲□◆□○◇□□□●●○● ◆□▲           
+         □ ▲◇◇◇□◆  ◆○◇◆ □△◆□▲         
+        ○○◇◇◇◇●□       ○◇○□◇△▲        
+       ◆■◇□◇◇○◇△○▲       △ □◇◇○       
+      ◇ ●◇◇■ ◆■○●◇○       ▲▲◇◇◇       
+       ◆○■◇  ○◆◇○◇○◇       □□○◇◇      
+      ◆■△◇◇   ●□△◇□◇◆      ■ ◇◇◇      
+       △■◇□ ▲■◆△◇○△◇◇●●◆◇◇●●□○◇◇      
+      ◇▲■◇◇●△◇◇△■◇△○◇◇○○▲●▲●○◇○◇      
+       ○ ●○△□◇□◆△  □□□□□□■■◆△◇◇       
+       ■◆ △◆△◆◆         ▲■▲△●△■       
+        ■●▲□○○○◆      ▲○◇▲□△□■        
+          ○◆■○◇□○●◇◆◆◆□□◇□□△          
+            ■◇□■◇△◇◆◆▲◆●□□            
+