Fix error reporting

[johnpryan]

This adds support for displaying Dart or Flutter error messages in console and lets you drag to adjust the size. If an error message has been received since the user last cleared the console, we show that. If there's only normal output, we show that.

One potential issue with this approach: If there's an error being displayed, you can't go back to view view console output until you clear the console. We could add a tab bar to switch between the two, but this was the best I could do without adding extra UI components that take up space.

fixes #3008
fixes #3148

[github-actions]

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pkgs/dartpad_ui/lib/enable_gen_ai.dart
pkgs/samples/lib/brick_breaker.dart
pkgs/samples/lib/fibonacci.dart
pkgs/samples/lib/google_ai.dart
pkgs/samples/lib/hello_world.dart
pkgs/samples/lib/main.dart
pkgs/samples/lib/sunflower.dart