Introduce Orientation concept to List

When the Orientation is Bottom, we paint elements from the bottom of the list when underflowing and express scroll position relative to the bottom. In either orientation, when inserting elements outside the visible area, we adjust the scroll position as needed to keep the visible elements stable.

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-Authored-By: Antonio Scandurra <me@as-cii.com>

zed/src/chat_panel.rs

@@ -38,7 +38,7 @@ impl ChatPanel {
         let mut this = Self {
             channel_list,
             active_channel: None,
-            messages: ListState::new(Vec::new()),
+            messages: ListState::new(Vec::new(), Orientation::Bottom),
             input_editor,
             settings,
         };
@@ -82,6 +82,7 @@ impl ChatPanel {
                     .cursor::<(), ()>()
                     .map(|m| self.render_message(m))
                     .collect(),
+                Orientation::Bottom,
             );
             self.active_channel = Some((channel, subscription));
         }