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_28.txt

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