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_2.txt

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