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_1.txt

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