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_10.txt

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