Sticky multibuffer headers (#22391)

https://github.com/user-attachments/assets/92cc5ff7-d8be-4e4b-ac6e-68eb310fffce

Release Notes:

- Multibuffer headers will now stick to the top of the viewport as you
scroll
- Added support for expanding diagnostic excerpts

---------

Co-authored-by: Michael <michael@zed.dev>

crates/editor/src/display_map.rs

@@ -32,6 +32,7 @@ use crate::{
 pub use block_map::{
     Block, BlockBufferRows, BlockChunks as DisplayChunks, BlockContext, BlockId, BlockMap,
     BlockPlacement, BlockPoint, BlockProperties, BlockStyle, CustomBlockId, RenderBlock,
+    StickyHeaderExcerpt,
 };
 use block_map::{BlockRow, BlockSnapshot};
 use collections::{HashMap, HashSet};
@@ -1105,6 +1106,10 @@ impl DisplaySnapshot {
             .map(|(row, block)| (DisplayRow(row), block))
     }
 
+    pub fn sticky_header_excerpt(&self, row: DisplayRow) -> Option<StickyHeaderExcerpt<'_>> {
+        self.block_snapshot.sticky_header_excerpt(row.0)
+    }
+
     pub fn block_for_id(&self, id: BlockId) -> Option<Block> {
         self.block_snapshot.block_for_id(id)
     }