Add an indicator to the channel chat to see all the messages that you missed (#7781)

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.

crates/channel/src/channel_store.rs

@@ -348,6 +348,21 @@ impl ChannelStore {
             .is_some_and(|state| state.has_new_messages())
     }
 
+    pub fn last_acknowledge_message_id(&self, channel_id: ChannelId) -> Option<u64> {
+        self.channel_states.get(&channel_id).and_then(|state| {
+            if let Some(last_message_id) = state.latest_chat_message {
+                if state
+                    .last_acknowledged_message_id()
+                    .is_some_and(|id| id < last_message_id)
+                {
+                    return state.last_acknowledged_message_id();
+                }
+            }
+
+            None
+        })
+    }
+
     pub fn acknowledge_message_id(
         &mut self,
         channel_id: ChannelId,
@@ -1152,6 +1167,10 @@ impl ChannelState {
         })
     }
 
+    fn last_acknowledged_message_id(&self) -> Option<u64> {
+        self.observed_chat_message
+    }
+
     fn acknowledge_message_id(&mut self, message_id: u64) {
         let observed = self.observed_chat_message.get_or_insert(message_id);
         *observed = (*observed).max(message_id);