[Accessibility] Add flag to app.bsky.feed.defs#feedViewPost to indicate author thread

[seboslaw]

Is your feature request related to a problem? Please describe.

Users who depend on screen readers can navigate the timeline. However, there is no indication for them whether the destination of a click on a cell is a single post or an entire thread created by the user.
For the latter case I have build an unroll function / view in to my app Skeets, which users with VoiceOver can use to easily read an entire thread of sequential posts created by the same author: https://bsky.app/profile/skeetsapp.com/post/3lardosponk2z. It would be very nice to be able to indicate to these users which posts will provide them with this deep dive and which don't. That way they could open this thread unroll view straight from the timeline.

Describe the solution you'd like

add a parameter to feedViewPost like "isPartOfAuthorThread:Bool". then I could just load the root and unroll