refactor(Playlist): Ignore ContinuationItem nodes from SectionList#contents

[LuanRT]

This should address some issues where the library would fetch the wrong continuation or even empty continuations.

NOTE: The solution in 987f506 no longer applies after this PR is merged, as empty continuations were all in SectionList#contents. If you had any code in place to handle that error, it is safe to remove it after this PR is merged.

Other related changes

1. Added style to PlaylistVideo. This helps identifying if the video is just a recommendation (PLAYLIST_VIDEO_RENDERER_STYLE_RECOMMENDED_VIDEO) or part of the playlist.

2. Playlist#items can only return PlaylistVideo nodes now. Its return type isObservedArray<PlaylistVideo>.

[iBicha]

LGTM, thanks for the quick fix!

[absidue]

Looks like this broke support for shorts only playlists (they use ReelItems instead of PlaylistVideos), as this code assumes that playlists only contain PlaylistVideos. It results in the following error being thrown in the items getter: Error: Expected node of any type PlaylistVideo, got ReelItem

As I don't know what the equivalent for the style attribute is for ReelItems or if recommended ones even appear in playlists that only contain shorts, I won't be able to make a pull request to fix this myself (as discussed in the other thread video recommendations only show up with you are logged into YouTube).

Here is an example of a shorts only playlist (the auto-generated shorts playlist for the @YouTubeMusic YouTube channel): UUSHStaiwu-FAgp_RC_tBiLh9A