Fix scroll jitter by ignoring autoscroll events if following (#4216)

When following someone else we saw jitter because
`ScrollPositionChanged` events were battling `SelectionsChanged` events,
both of which were scrolling the viewport.

This ignores the ScrollPositionChanged if autoscrolling is on.

Release Notes:

- Fixed jittery scrolling when following someone else that had a bigger
viewport.

crates/editor/src/items.rs

@@ -277,7 +277,7 @@ impl FollowableItem for Editor {
                         .extend(ids.iter().map(ExcerptId::to_proto));
                     true
                 }
-                EditorEvent::ScrollPositionChanged { .. } => {
+                EditorEvent::ScrollPositionChanged { autoscroll, .. } if !autoscroll => {
                     let scroll_anchor = self.scroll_manager.anchor();
                     update.scroll_top_anchor = Some(serialize_anchor(&scroll_anchor.anchor));
                     update.scroll_x = scroll_anchor.offset.x;