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_3.txt

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