feat: Allow pasting newlines (#866)

Noticed that when pasting multi-line blocks, each newline was treated
like a new submission.
Update tui to handle Paste directly and map newlines to shift+enter.

# Test

Copied this into clipboard:
```
Do nothing.
Explain this repo to me.
```

Pasted in and saw multi-line input. Hitting Enter then submitted the
full block.

codex-rs/tui/src/tui.rs

@@ -2,7 +2,9 @@ use std::io::Stdout;
 use std::io::stdout;
 use std::io::{self};
 
+use crossterm::event::DisableBracketedPaste;
 use crossterm::event::DisableMouseCapture;
+use crossterm::event::EnableBracketedPaste;
 use crossterm::event::EnableMouseCapture;
 use ratatui::Terminal;
 use ratatui::backend::CrosstermBackend;
@@ -19,6 +21,7 @@ pub type Tui = Terminal<CrosstermBackend<Stdout>>;
 pub fn init() -> io::Result<Tui> {
     execute!(stdout(), EnterAlternateScreen)?;
     execute!(stdout(), EnableMouseCapture)?;
+    execute!(stdout(), EnableBracketedPaste)?;
     enable_raw_mode()?;
     set_panic_hook();
     Terminal::new(CrosstermBackend::new(stdout()))
@@ -35,6 +38,7 @@ fn set_panic_hook() {
 /// Restore the terminal to its original state
 pub fn restore() -> io::Result<()> {
     execute!(stdout(), DisableMouseCapture)?;
+    execute!(stdout(), DisableBracketedPaste)?;
     execute!(stdout(), LeaveAlternateScreen)?;
     disable_raw_mode()?;
     Ok(())