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_29.txt

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