This PR adds support for extracting YouTube live chat messages and exposing them through the existing comments API. When a live stream has regular comments disabled, the extractor now falls back to fetching live chat messages instead.

Changes

• CommentsExtractor / CommentsInfo: Added isLiveChat() flag to distinguish live chat from regular comments.
• YoutubeCommentsExtractor: Added findLiveChatContinuation() and fetchLiveChat() to retrieve live chat via the live_chat/get_live_chat endpoint.
• YoutubeLiveChatInfoItemExtractor: New extractor that maps liveChatTextMessageRenderer JSON to CommentsInfoItem, with proper emoji run handling.
• Page routing: Live chat continuations are marked with a live_chat page identifier so getPage() routes correctly on subsequent extractor instances.

Implementation notes

The approach is based on the live chat implementation in PipePipe, adapted to fit the NewPipe Extractor architecture. Key adaptations include:

• Reusing the existing CommentsInfoItem type instead of introducing a separate live chat item class.
• Proper isLiveStream flag handling to avoid hitting the replay endpoint on fresh extractors.
• Emoji extraction with fallback chain: emojiId → shortcuts[] → searchTerms[] → [emoji].

Checklist

• I carefully read the contribution guidelines and agree to them.
• I have tested the API against NewPipe.
• I agree to create a pull request for NewPipe as soon as possible to make it compatible with the changed API.