Support specifying attributes to DartPad

[parlough]

Enables the mechanism for adding a title attribute to DartPad code blocks.

```dartpad run="true" title="The classic hello world program in Dart!" theme="dark"
void main() {
  print('Hello world!');
}
```

Contributes to #5251

@atsansone After this lands, feel free to add titles to the embedded DartPads. I'd be happy to review!

[dart-github-bot]

Visit the preview URL for this PR (updated for commit 42287ec):

https://dart-dev--pr5884-feat-support-specifiying-dartpad-attr-sv5ci0mm.web.app

[atsansone]

LGTM % 2 questions:

1. Why do we not set height on DartPad includes any more?
2. Why default to light theme?

[parlough]

Thanks for the review and questions @atsansone!

1. Why do we not set height on DartPad includes any more?

The new embed mechanism hasn't supported it, so removing the mentions to avoid misconceptions that it is supported. A lot of them time, they run-dartpad portions were copied from elsewhere without paying too much attention I think. If there's a strong desire to bring height adjustment back, open an issue, and I can definitely do so!

2. Why default to light theme?

That's just what we've always done for embeds since the site body is light theme. Then in codelabs, dark theme indicates it's a conclusion exercise.