This pull requests add the following features:
- Show indicator before first unseen message
- Scroll to last unseen message

<img width="241" alt="Screenshot 2024-02-14 at 18 10 35"
src="https://github.com/zed-industries/zed/assets/62463826/ca396daf-7102-4eac-ae50-7d0b5ba9b6d5">


https://github.com/zed-industries/zed/assets/62463826/3a5c4afb-aea7-4e7b-98f6-515c027ef83b

### Questions: 
1. Should we hide the indicator after a couple of seconds? Now the
indicator will hide when you close/reopen the channel chat, because when
the last unseen channel message ID is not smaller than the last message
we will not show it.

Release Notes:
- Added unseen messages indicator for the channel chat.