Streaming markdown (#1920)

We wait until we have an entire newline, then format it with markdown and stream in to the UI. This reduces time to first token but is the right thing to do with our current rendering model IMO. Also lets us add word wrapping!

codex-rs/tui/src/app_event.rs

@@ -50,6 +50,10 @@ pub(crate) enum AppEvent {
 
     InsertHistory(Vec<Line<'static>>),
 
+    StartCommitAnimation,
+    StopCommitAnimation,
+    CommitTick,
+
     /// Onboarding: result of login_with_chatgpt.
     OnboardingAuthComplete(Result<(), String>),
     OnboardingComplete(ChatWidgetArgs),