New excerpt controls (#24428)

Release Notes:

- Multibuffers now use less vertical space for excerpt boundaries.
Additionally the expand up/down arrows are hidden at the start and end
of the buffers

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Zed AI <claude-3.5-sonnet@zed.dev>

crates/ui/src/components/icon.rs

@@ -192,6 +192,8 @@ pub enum IconName {
     ExpandVertical,
     Exit,
     ExternalLink,
+    ExpandUp,
+    ExpandDown,
     Eye,
     File,
     FileCode,