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_30.txt

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+                                      
+                 ○◉ ●●●●              
+               ○◉◉··○●●○○             
+                ●○·●○○·●●○            
+              ··○·◉·○○·○ ○●           
+              ○●···○·●·●  ·           
+             ·●·○●  ●····◉·           
+             ○◉···○○ ○◉○· ○·          
+              · ●●○·●· ·◉  ·          
+              ·○ ○·○·○◉·●  ·          
+              ··○·○○·· ●●  ·          
+              ○··◉○○●···●··           
+              ·●◉◉●◉●○··  ·           
+              ○·○··◉●··●·◉            
+               ●○●●○○◉·●◉●            
+                ○◉◉·○○○◉●             
+                  ····