Add isLive indicator to related videos

[PrestonN]

Hello!

I've seen some edge cases where some of the related videos from a video I'm watching are actually live videos. These videos will have an undefined length_seconds so I can use that if I need to for determining if that video is live is now, though an isLive indicator might be helpful as well if there are edge cases where the length_seconds returns undefined even on non-live videos.

Here's an example video that should probably help with this: https://www.youtube.com/watch?v=DDU-rZs-Ic4

I'm not expecting this to be a priority, just figured it might be worth adding at some point.